Yes, you are right, it is very uncommon to greet the same person more than once a day. If you do this, you can hear: 'you have already greeted me today". If you happen to see somebody again, you shouldn't say anything, just smile, greet them with a nod of the head or wave your hand.

Здравствуйте is a formal form. Informal is здравствуй, but it isn't used so often as привет. And yes, you can use it many times a day. There is a frase "И снова здравствуй(те)". It has a shade of meaning like accidental remeeting is funny for you.

if i come to somebody the second time i usually say - я опять к тебе ( к вам) , ещё раз я . But if it is attention that refers to everyday work , i prefer speak about deal without any second introduction

I`d advise "как дела?". It sounds polite enough and neutral enough. But should not say it each 5 minutes )) 2-3 times a day is enough.