I believe you will get more use out of 30 dummy rounds than you will a ruined firing pin. BTW, you found out what could happen if you assemble the BCG without the firing pin being captured by the FP retaining pin, except if you were firing live rounds the gun may have blown up. When messing with the firing pin, be sure its captured.
Anyway, any reloader will be glad to take your fired cases and stick bullets in them - they are always good to have for checking out new mags, new rifles, etc. The only thing you have to worry about when using them indoors is breaking a window when you yank the charging handle to extract - they will fly out pretty hard. And that's what you want to do - yank the handle to extract and let it fly home to load, to simulate what happens when firing. Nothing is learned when doing it 'easy' except it usually will cause a jam.