Let's take this bit by bit:
What if the video of a Weeping Angel is observed let's say from the future or the past?
This is confusing, because normal videos are always of the past. Are you talking about looking through a time portal to observe a video of an angel?
Does the Angel still get frozen because it's being observed however the time setting is different?
Ye-es but as we learned in the episode with the video of the angel, nothing is to stop the angel from, for example, moving between the frames of the recording. And since even the image of an angel is an angel (such as reflecting off of your eyeballs apparently) you're still screwed.
Besides which, time begins to get wonky down on the quantum level.
For example: An Angel attacks your friend. He recorded it and you wanna see what happened. So the Angel is being observed through the tape. A quantum mechanic lock works only when it's not being observed by a living creature.
So the setup from the episode where an angel is being observed through a recording, okay.
So does the Angel still "kill" your friend?
Sorry, are we going with the assumption that your friend escaped and handed you the recording, or that you came afterward and found a cell phone laying on the ground? Because this seems to clearly answer the question. You observing the recording does not affect whether or not your friend was attacked.
I think you are confusing looking at a video of the past with directly affecting the past.
As I understand it, a video of an angel is its own entity. To the best of my knowledge, observing a video of an angel does not affect the angel who was being recorded however long ago it was.
Now, is the angel on the tape a separate entity?
That's not entirely clear. It does seem clear, however, that watching the tape of an angel causes the tape of the angel to transfer into the person doing the viewing. Is this affected if, say, two people watch the video? That is, does only one person get "infected" by the angel or do both? We just don't know.
In fact, that more or less summarizes this question- we haven't seen it, so we don't know. But there is also no reason to think that observing a video of an angel will affect it at the time either, so I would lean towards no until we have any sort of proof otherwise.