Fragile screws break after screwing them?

hello everyone,

so here's my problem: my iphone 4's lock button was stuck, so i took the device apart to see what was wrong. i fixed that issue (by improvising somehow), but when i put the iphone back together, the heads of some screws broke while turning them. fortunately they were all already in place, but i'm scared that i won't be able to disassemble the device again. and since my solution for the lock button was only improvised, i'm afraid the next time i'll have to repair it won't be too far away.

so here are my questions: is there any way to take out broken screws? what could i possibly be doing wrong that they brake? or ist it just normal that these parts break after a while?

and if i can't open it up again, will i still be able to send in my device, even though i took off the warranty sticker? since my warranty ran out anyway, i'd have to pay so or so, but someone told me apple wouldn't fix it (in that case) anymore, not even for money. is that true?

sorry for so many questions!