Hello Akiru! Stripped screws can be a bugger, but you do have some options. I could outline some of your options here, but iFixit has a good article for this one. Go ahead and take a look at the article on iFixit titled "How to Remove a Stripped Screw", it should help you out.
Hello! Unfortunately replacement parts for this particular model are difficult to find. There seems to be some replacement parts for the XL; however, I don't think that they are the compatible with the regular 3ds. Additionally, slot replacements are sold widely; however, it would require difficult smd rework. Your best bet is to find a New 3ds 2015 that is being sold for parts on ebay that has a working gamecard reader and use that as a replacement.