Will it be ok to do payments/offer services in grin in the sections that were exclusively marked for bitcoin?
No.
Can GRIN do this? Can GRIN compete with VISA and remain decentralized and immutable with a fixed supply and no trust in 3rd parties?
s/fixed supply/predictable supply/Yes, that combined with its privacy is what makes it so exciting. You can probably achieve even greater scaling with BTC+LN, and I consider that
quite likely to be what the economy
actually goes toward, but grin is a very different and interesting alternative approach.