I'm not sure there is a 'correct' thing to say for all of these, it will vary by region/class. I think 'pardon' is considered 'lower class', so if that would bother you maybe avoid that. 'What' I think is upper class but sounds rude to me.
Personally I say: 'I'm sorry?' for 'I didn't hear you' 'Sorry!' for 'Forgive me for bumping into you' I'd probably just say pretty much the same as 'I dont understand what you just said, can you explain it please?' 'Oops!' for 'Oh dear I just dropped something'
'huh' drove me to screaming point as well, but it is worth the slight effort of bending down and talking eye to eye with your DC. Sorry to seem patronizing, which I don't intend to, but my DS stopped saying 'huh' altogether when I started to do this, after reading in a book that boys particularly need this strategy. DD never did it a all
For 'please repeat' I say 'Can you say that again please?' Bumping into people I say 'sorry' 'I don't understand' I would say just that For dropping something I would say 'bugger!'
The only time I would say 'pardon' is after a burp and then I would say 'I do beg your pardon'