as for installation of the BSE on torch esp an all body one, not sure. I've always just gone for the protector on the screen itself and haven't had a problem nor have my friends with the installation or the service they provided i.e its not hard at all
protective film? if you're talking about just the normal stuff you get at your local shop, probably not. this stuff is pretty damn resistant to a lot of pressure when it comes to trying to scratch the damn thing. hopefully that answers your question.
For the most part, you wont notice it on there but it has never ruined the experience for me. The installation is quite straight forward and it seems very similar to the process used by ZAGG's invisibleSHIELD except these guys (BSE) offer their skins at a better price if not better service. The process being (if they haven't found some awesome new way) is by adding a bit of solution to the device and slapping it right on there.
I've never had any bubbles or problems putting it on but that's not to say it won't ever happen. order 2 if you feel like you might not feel so confident - get it right the first time and you'll have one to give to a friend cause you know, you'll throw this is in as a sweetener and you can be bbm buddies, haha!
I've got one on my zen vision m that i bought in 2006. it still looks pretty damn good now with heavy usage and im quite surprise its lasted this long! My friend took one off his ipod and it looked as good as ever... for an ipod haha.
I can't see it being a problem with a removable battery. it'll probably come in a few cut out sheets to place it on yourself. more work yes but hardly something i'd pull a muscle from - Knowing from past experience it wont hinder your experience and if you got a question about it, they really are accommodating.
it would complement an otterbox but seeing as i dont really care for cases, im quite happy with the use i get out of a skin such as this and whilst it would be saving you quite a bit, for me its all about the protection of the screen! It's not something that will protect it from heavy drops as there's no 'cushioning' between the skin but by putting it in your pocket amongst keys and coins etc I can safely say that you'll be right with just the skin. (I've dropped devices with the skin on and im thankful for it being on there since a device sliding across gravel just isn't going to look pretty unless you've got the protector on but luckily )
If i had one, i wouldn't worry to much about carrying a case but certain cases are definitely eye catching and for that extra assurance, i wouldn't say to you to not get it.