What are the limits on transferring objects during fireplace-to-fireplace conversations? Is it exploitable?

Question

Early on in chapter 11 of Goblet of Fire, there is a conversation between the Weasley parents and Mr. Diggory. Mr. Diggory's head is appearing in their fire and he is using this to talk. During this exchange, we get this line:

Mrs. Weasley took a piece of buttered toast from a stack on the kitchen table, put it into the fire tongs, and transferred it into Mr. Diggory’s mouth.

This shows that objects can be transferred during fireplace-to-fireplace conversations. Are there any limits that are established for this?

What's made me concerned about this is the fireplace-to-fireplace conversation between Harry and Sirius near the end of chapter 14 of Order of the Phoenix. As Harry was in the Gryffindor common room during this, it's opened up two uncomfortable questions that I'm hoping will be addressed by the limitations:

• How can Harry and Sirius be struggling to communicate via letter if all that they need to do is appear in the fireplace for a moment and throw a letter in? Surveillance isn't a major issue; You only need a moment to catch a letter and therefore you can safely do this in the early morning. Furthermore, Harry's hypothetical reply - throwing a letter in to a fire - is not very suspicious.
• How can Harry consider the Gryffindor common room safe? It has a fireplace that seemingly anyone can send objects through. The vulnerabilities are blatant and really should've been mentioned to the boy who has had hidden murderers supposedly trying to kill him for two or three years in a row (Sirius in third year and a then-unknown person in fourth).

Admittedly, after the scene at the end of chapter 17, where Professor Umbridge's ringed hand appears to try to grab Sirius' hair through the fire, I'm not filled with optimism.

Answer 1

Why didn't Harry and Sirius give letters to each other through the Floo Network?

To answer this, I will assume that we are comparing passing letters through Floo and just talking face-to-face through Floo.

Security

• Passing letters seems to have the upper hand here as appearing quickly in the fireplace, passing a quick letter when nobody is seeing then disappearing takes very little time There is also a lesser chance of being caught by someone who is trying to monitor the fireplace.
• Talking face-to-face is not very secure. Someone with the authority to monitor the fireplace can easily catch the person trying to talk and/or passing whatever they need to.

Enchantments

• It is certainly possible that appearing in the fireplace is possible but getting out of it, or passing materials to another person through it is forbidden by magic in the dormitories.
• Here, talking face-to-face seems to be the better option. There is no need to pass materials so no restrictions.

Convenience of replying

• If we consider that passing letters through Floo is just like passing letters through owls, but just with that extra bit of privacy, then we are burdened by the fact that whatever we want the person to answer, we have to write that down, make sure that no one is spying, pass the letter on (making sure that Rule 2 does not apply here), and then anxiously wait for a reply.

• Face-to-face conversations has the upper hand here as well. Since we can talk to each other and instantly get the reply, we don't need to wait.

Hence, talking face-to-face seems like better option.

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Why didn't Harry realise that if Sirius could visit him, anyone could?

Consider the following facts :-

• It is, like I have said above, very possible that there are protections against someone trying to import and export materials through the fireplace. Although this is not proven anywhere in the books, Dumbledore wasn't much of a fool either.
• Dumbledore was match enough for the most powerful witches and wizards known at the time. It is, again, possible that he had put enchantments to prevent against the possibility that Harry be attacked by some random evil wizard.
• Most people at the time (when he was young) who were trying to kill him were afraid of Dumbledore and were less in number as it is. Even a very large group of them, I think, would not be enough to take on Dumbledore.
• When Voldy came back from his more-or-less-a-ghost state, Harry was smart enough to know what was good for him and what was not. He also had a good idea about who were his enemies and the people he could rely on.
• He also probably did realise that anyone could visit him, but he also knew that one of the most powerful wizards of all time was on his side to protect him if the time arose to do so.

Answer 2

Why not send letters through the Floo Network?

Below are all possibilities, as there is no concrete answer I can find in the books or in Canon.

Coincidences

Say someone else happens to be walking by the fireplace, and they feel a letter hit their foot. They will pick up the letter and examine it to find "Grimmauld Place" on it. Harry will tell them the letter is for him and the person will either give / not give it to him and forget / not forget the letter and the address.

In this first scenario, it will depend on the niceness of the person and their ability to remember things. Also, if the person gossips, Harry will be in trouble.

The letter may be intercepted. Although Sirius's head was not, it is less suspicious to have a head going through the Floo Network than a letter. Heads go around all the time, and letters are a little less frequent. And if the letters are to Gryffindor and going to or from some random place the Ministry's never heard of, they'll be suspicious.

This scenario will depend on the attentiveness of the Ministry worker and how much they are watching the Gryffindor fireplace. Fudge will have put someone loyal to him in this job, undoubtedly.

Accidents

The letter burns.

No one knows if this will happen. After all, wizards are magic and letters are not. It would be simple to add a non-flammable charm to the letter, though.

The letter goes out of the wrong fireplace.

There are also quite a few places this letter can go! Floo powder can misinterpret the speaker's words (as seen in Harry's second year at Hogwarts). It would be less worrisome if it was the Ravenclaw, Hufflepuff, or Headmaster's fireplace. But the Slytherin fireplace would be devastating.

It's happening!

The letter needs a body part to go with it.

The toast went with Mr. Diggory's head. We don't know if the toast needs the head to go through. So Sirius may have to stick his hand through with the letter and cause a lot of screaming and alarm.

Fireplaces don't like letters.

Fireplaces may have tempers. It is the magic world after all. Either that or fireplaces in the Floo Network are enchanted to not allow letters through.

Owls are better.

They're magical pets. I'm not saying fireplaces are bad pets, it's just owls are extra special in the wizarding world. Many wizards prefer owls. Hedwig would also be mad if Harry chose a fireplace over her.

If Floo network, then not owls.

Continuing from the "Owls are better". If the Floo Network were for mail, magic owls wouldn't exist as pets. Wizards would use the Floo Network as new technology and let them roam wild. Only a few would use the owls because owls take longer to go there and back.

Conclusion

Most of this is speculation, but through deep thinking and thought it is possible to realize all these scenarios are possible to some point. The main focus must be put on Fudge though. Below is the point that I consider to be the deciding factor.

Umbridge was able to find Sirius by watching Harry's mail. If Harry's mail was sent through the Floo Network (controlled by the Ministry), it would end in the same result with possibly more disastrous consequences. Somehow, despite the secrecy behind Sirius's first meeting with Harry, Umbridge figured out that he was using the Floo Network. Continuing to use the Floo Network will allow the Ministry to read all of Harry's letters without him knowing, perhaps for the entire year. The Ministry might even be able to use the information gathered to capture Sirius, despite the Fidelius Charm, which can be broken.

Therefore, neither Harry nor Sirius sent letters through the Floo Network.

Answer 3

I think something that you are holding to or wearing while using the floo powder is acceptable. Then again you could place different levels of restrictions or surveillance.