I really can't offer any "material" rewards to pledgers (t-shirts, mugs, etc.). [...] What would be good pledge rewards that would actually give value (aside of full copy/early access to the game, because it's given at this point) and wouldn't cost me too much or nothing at all
"Your stuff in the game" is the bread and butter type of reward for larger pledge levels. It makes sense, as crowdfunding on principle is supposed to be about helping someone create something, and paying a premium to leave your mark on something you think looks cool is a good system.
The typical reward progression is typically something like-
Thanks > Digital Copy > Digital Copy + OST > Physical Copy and/or physical doodads > stuff in the game
2. How to put realistic campaign goal that would provide me funds I need, yet would be achievable?
How many funds do you need? There's your goal. Do take into account website fees and taxes, though. Add those onto what you need.
3. Is flexible funding a good idea? I plan to finish game "no matter what", but needs some funds to do so (licenses, hiring actual musician to create OST, perhaps voice actor for the trailer - art and rest still would be done by me). But I feel that it could deter some people.
Flexible Funding is a great concept [for
some things] but I think projects on IGG are going to have a harder time than on KS in general. IGG has even less of a built-in audience than KS does (not that you should be relying on KS's audience alone to make or break you, mind) so you will need to pull even more of your weight. That said if your goal is low like 1K you'd probably be able to make it work on IGG.
You should try to hype up your game before the actual campaign. This was a big mistake I personally made. I came out of nowhere with my KS when I should have been building a fanbase up to the KS's launch.
Also a side question: Since I plan to start campaign when I have something playable, is putting short demo of the game, say 5 levels alongside campaign a good idea? Or would it act as deterrent to potential players?
That's a personal choice, there are lots of people who choose to go either way. Personally I did not offer a demo and many other games on KS don't. This is pretty much because I didn't want to take away from the magic of the game from when it's released proper... like, imagine if people could play the first couple dungeons of Ocarina of Time a year before it came out. That would have dulled a lot of its impact imo :p