I am wondering, can I start RNGing only with these 2 things as a form of frame-advance and seed verification?
Yes you can.
Second, I was like extremely lucky to encounter a shiny rattata while going back to prof. Elm to show him the hatched togepi. So with this rattata I looked for my SID number. From what I know so far, with this number, I can RNG to hatch/encounter shiny pokemon too, right?
If you want good IVs, stick with breeding. But yes, you'll be able to hatch and catch shiny Pokemon now that you have your SID (but the captured Pokemon probably won't have good IVs since you did't abuse your ID/SID).
My friend says you can repeat the event and get a second egg if you have 2 different arceus (in this case the hall of origin and the event arceus, they have different ids) is this true?
It's partly true. The Hall of Origin Arceus was never released, so you can't use that to activate the event. However, if you have the Toys-R-Us Arceus and some other Arceus event (I think it was Michina or something, I can't remember) you can activate the even twice.
Anyways the reason i am asking about the Arceus event is because I would like to know if you can RNG palkia/dialga/giratina without having any roaming pokemon (since i just started the game and i am on the 3rd gym, i suppose i have no roaming pokemon to perform the seed verification)
Yes, you can RNG them without having roaming pokemon. You just have to save outside of the entrance to the ruins and use Irwin to verify your seed and advance your frame.
PD: can you calibrate somewhere and somewhow early in the game? (mt. silver is kind of far)
You don't need to calibrate, just use Eon Timer and it will calibrate for you.