1) Most sites are designed for Chrome these days, and our customers also use Chrome or Edge. I get not wanting to support Google itself, but Chromium has become the de-facto standard at this point, and Firefox is actively declining with 6% desktop and 3% overall share.
2) This is a preference thing for sure, I'm a fan of vertical tabs and was previously a Vivaldi user. I really like the design of pinned tabs too, it makes me actually use them. This is a workflow thing and I'd just avoid Arc if you don't like vertical tabs.
3) Who knows, Vivaldi and Brave manage to stick around. The Google integration money doesn't seem to be bad. They also don't need their engineering team to encompass developing the rendering engine, they can focus on the UI.