Spoiler for About those questions:
Disclaimer: What I understand so far might be wrong, as I don't think I understand much (if any) jap...so don't take it as the truth.
Near the river, Chiaki was saying that he has to go back to his time. Makoto gets all tsudere'd up, and asks him to just go. Then she cries. When Chiaki comes back, he tells her to look out for a time traveller. Makoto says she will wait for him now, tomorrow, as long as it takes.
Ok, to your questions:
About the painting, Chiaki said that during his time, the arts (had something wrong with them) and he very much wanted to see the works of masters from this time, thus he came to this location, this time to see them. But alas, the one he wanted to see (No.12 or something) seemed like it had been taken down from the viewing gallery, as it had to be protected or something.
Yup, the ending was possibly meant to be bittersweet, with a tinge of hope mixed in. I'm thinking that she will wait for him
Talking about Makoto not being able to leap forward in time - maybe it affects all users? Chiaki could go to Makoto's time because it was in his past?
About the manga, gotta ask NeoSam
Ya, the voice acting was really good - especially the students, as they really did sound like students.
Nice backgrounds and 3D effects (and to a lesser extent character animation), coupled with the VAs, and most of all the storyline, have made this one of the better anime movies.
I kinda got a bit hyped up about the whole show for the last few days, that I tried my hand at subbing it. However, 10 hours later, and lots of cursing and swearing, I can only present the first 5 mins:
http://www.mediafire.com/?82mk2gj2gwm
I really take my hat off to all subbers - the whole process is certainly not easy. A lot of the dialog seemed like it wasn't pure standard japanese - like slurs of certain words and short forms were used. Makes for hell when trying to decipher bursts of words.
BTW, I fully expect "Mass Naked Events"-level type of errors in the sub. Hell, some sentences were even left out since I could understand a word.
I think I had better leave the subbing to the pros at Saizen