- They could have, but I do not think it would have been worth the risks.
- There are a lot of unknowns here, but I do not think that matters. The only way their plan really can work is if they subtly sneak in. Even still, I do not understand the end goal... It is possible, but I think such a plan is doomed to failure.
- The real question is why didn't the majority of the Wraith land their ships at the bottom of random planet oceans and go back to sleep after they found out Earth was inaccessible?
For the Wraith, there are a lot of unknowns here. What we do know is how specific the information they knew was. They knew they needed to be "looking" when the Atlantis gate was activated to go to the bridge, to hack the code to gate in, secure the station of course, and that they would need to bypass Earth's extensive defenses. How did they know all of this? It must have come from a captured team member that was taken AFTER or maybe DURING the bridge's construction. Given this, how much else did the aforementioned prisoner tell them?
Given they have some information, let's say that they know the Milky Way situation entirely and every possible Wraith cruiser left together for the promised land; knowing full well the war with the Ori was ended and they may need their combined force to defeat a (nearly) unified Milky Way galaxy.
So they all travel along the gate path with nearly everyone but the care takers back in stasis for the journey. They bring a bunch of "snacks" with them given that this trip will take a while for them. Wikis online state that it takes them weeks to travel across Pegasus, let alone the time between galaxies, without ZPM power. The Daedalus in Season 5 , Episode 2 "The Seed" is said to be out of contact for three weeks, but I have also read 18 days for the trip as well; these are relatively the same and either of these is significantly less than the Wraith. In addition, given the ease of travel between the galaxies the daily or weekly check-ins are happening from one side or the other, thanks to the bridge to at least send reports back home for review.
Now, let's say that Atlantis never catches on to the fact that they have all the sudden stopped seeing the Wraith and that no other planet they have communication with has not mentioned it either, so they never think to put it in a report.
Once the Wraith fleet is detected by the bridge station, they are outed and Earth knows what is coming and has time to prepare. Considering the Wraith were detected, whether it is one ship, all ships, or somewhere in between that has decided to make the trip there is a good chance that as soon as they drop out of light-speed in the Milky Way they will have an armada waiting for them.
But let's say the bridge station doesn't have a chance to get the word out for some reason. Destroyed or whatever. When they miss check in, Earth sends the Daedalus (or another ship) to find out why. This is really the only saving grace in this entire plan, in the beginning at least. I can not remember how many ships the Milky Way has at this point, but if the Daedalus was unable to relay the invasion for whatever reason the Wraith may be able to make it unannounced. Otherwise, if the status of Midway was sent, Earth has time to prepare again.
Once again, let's say that they out smart everyone. The Wraith dodge all the defense ships initially, no one believes Earth about the approaching danger, they were able to destroy the Daedalus, etc. The Wraith start culling and eating after their long trip. Even if it takes the Milky Way time to discover, years even that they have made it to the Milky Way and even if Wraith that are made the old fashioned way (assuming that is still possible) grow in both large numbers and mature quickly, once the war starts they will need at least a generation to begin replenishing their numbers.
Judging by human standards this would mean that the replacements would need to be at least ?10? to be able to operate the vessels and/or become efficient soldiers. We know that they do grow similarly to humans thanks to Season 2, Episode 7 "Instinct" when we see a pre-teen adolescent Wraith. It is possible there was heavy influence by her father, or that the soldiers grow differently, but we do not know that for sure.
At that point, under the best circumstances you have (however numerous) a limited number of Wraith against the entire combined forces of the Milky Way that has numerous abilities to manufacture more soldiers and ships. Unless the Wraith are lucky enough to get someone to ally with them or there is some serious infighting between the Milky Way factions, they are doomed to failure.
Without the guarantee of success I think that the Wraith would prefer to maintain the status-quo in Pegasus. Sure they may eat really well for a bit and have the ability to go out in a blaze of glory, but they want to survive as a species. That is why they all went to sleep after the war with Atlantis. The situation in Pegasus is survivable compared to the slim chance they would have assuming they survived the trip to the Milky Way.