Arr, here be SPOILERS
Janeway was never one to fully follow the rules throughout the series. She bent the rules of the Prime Directive countless times - and that was one thing I liked about her. I mean, face it, the Federation's "non-interference" policy, while it's good most of the time, doesn't apply to certain situations. Because of this, I don't think it's out of character for her ignore the Temporal Prime Directive..
I always figured that "future Admiral Janeway" probably knew she had to go back to that specific point in the timeline, because that was the crucial moment to defeat the Borg and get Voyager back.
Admiral Janeway giving tech to Captain Janeway? Not sure about that one. Never thought about it much before. It's been a while since I've seen it so I don't remember all the small details.
As for them not celebrating/setting foot on Earth; I liked it. It was simple and clean; looking out the viewscreen and finally seeing Earth. I don't think they needed some long, drawn out celebration at the end with streamers and music. Star Wars already did that...and it was cheesy.
You want a bad ending? Go watch Enterprise! (LOL) You'll think Voyager's ending is amazing after seeing that crappy episode. Oh, I forgot. We Enterprise fans aren't supposed to talk about that episode. It doesn't exist.