Using "Excuse me, ..." for asking questions
Solution 1:
Your "Excuse me?!" is an apology for saying "Excuse me", for interrupting them and claiming their time. (It's not "Sorry", because your interruption was deliberate and you would do it again under the same circumstances.)
The "Hey!" (or "What?", or "Yes?", or even "Hello?") response means "Okay, you've got my attention, what do you want?".
You are now already in conversation mode, so "Hello!" wouldn't be appropriate. Just get on with it and ask your question.
(In geek-talk, it is the equivalent of RTS/CTS, request-to-send/clear-to-send.)