Does upgrading RAM myself void my warranty for a future battery replacement?

If instructions for upgrading RAM are provided by Apple, doing so does NOT void your warranty. The rule of thumb for going higher than Apple recommends is to buy from a Vendor who can support the extended sizes they recommend, and will take it back if it does not work.

Apple does NOT approve of do-it-yourself battery replacement, however. The prices Apple quotes are for the battery, Installed, and with a modest warranty. Considering the difficulty of replacing glued-in batteries, their battery replacements are a bargain (if still available for your MacBook).

There are many different batteries available from ebay and amazon and other "off-Brand" resellers. Most are not dependable enough to take a chance on. I am really thrifty when it comes to this stuff -- but I recommend for batteries, you stick with Apple brand as long as they remain available.

Only if the computer was damaged in the process. Note that the limited warranty and AppleCare on a Mac sold in 2012 will have expired by now.

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I thought it doesn't void warranty but there are some people who had problems because of ram replacement.

Upgrading 16 RAM voids apple warranty - 'traceable if switched back'? | MacRumors Forums

I think it's way better if you get it replaced first before you upgrade your ram, but you could swap ram back for original one before battery replacement and everything will be ok.

Battery is glued so it's quite hard and risky to replace.

Stripped screws will not Void your warranty. But so many of these Macs have stripped or missing screws, they will typically just ignore the stripped or missing screws unless the bottom is falling off.

There is likely to be a good supply of those screws "in the back" where they do this service, and you should be able to get screws repaired/replaced at no cost. You usually have to ASK explicitly that they be repaired/replaced. The typical "fix" for stripped screw is to replace the screw (if the screw itself is stripped) or put some Loc-tite® (screw-glue) on it (if the socket is stripped).

Well it's just a screw if you get there you can ask about it like this guy:

https://forums.macrumors.com/threads/does-striping-a-screw-void-warranty.808182/

There should not be any problems

You can find MBP replacement screws on eBay for about $3