Solana is the largest of the cheap/fast networks - I’ve worked in Solana for a year - but Aptos and Sui also have solid tech.

You use Safari on your iPhone and unlock your wallet with FaceID.

Right now physical stores that take USDC are generally luxury goods marketplaces, physical art galleries, and PC hardware marketplaces.

No you can’t cancel transactions after you’ve made them, however there are escrow services that provide similar facilities for a small fee.

No it’s proof of stake so the environment is fine.