1. No, I dont see it why you get banned for just a save file that was properly signed like it was a legit one.
2. No bans were reported due to use of a resigned save file, its almost impossible to detect specially if that save file was done legitimately (by means of legitimately you played it without cheat), if you use a modded save file like in BOPS2 with lots of unlocked things and modified stats, then you deserved a ban. But hey, the main point here is use a clean save file and youre safe, it doesnt matter if you played it on CFW or not, as long as you played it without cheats using that save file is good.
3. No, the heck with that checking, everyone got banned already if they do that since with that kind of checking you can potentially ban legit users by 60%
3?. No save file are legit files that is created by the legit game it self, the only thing detects sony from your console in your PS3 is the trace of homebrew files on game folder, not in save folder
-- So the main point here is you are the king of the game as long as you have your ConsoleID in your hands, you can literally load any save files out there in the cyber space. You can just download the save file pack, resign it and do a plat on that game in less than a minute or hour.