It's interesting to see that the verb to go is also used to talk about the future in Welsh. I'm always surprised by how many languages have this construct which I would expect to be more idiomatic.
Aros is a verb here, right? So, i can say "dw i'n aros" to mean "I stay"?
Correct. Aros is a verb-noun meaning 'staying/to stay' and also 'waiting/to wait'
Dw i'n aros mewn gwesty - 'I am staying in a hotel' '
Dw i'n aros am Sioned - 'I am waiting for Sioned'
can aros mean stop too?
It can. As in heb aros - without stopping/waiting, or
Where I come from in Wales we often say"Are you going to stop?" in the sense"are you going to stay" eg Stop and have a cup of tea = Stay and have a cup of tea."
I don't like this method of Q&A here (ie. selecting the missing word) because you don't get to hear the sentence as a whole or see it in place. If as you selected it, it read the whole sentence, would be helpful.