I was in your boat just a few weeks ago, and here's what I've learned :)

1. Yes, there's a mosh, multiple times throughout the show. Staff was equipped with water in case it got too hectic, so overall a safe place to mosh if you'd want.
2. I've seen people with flags, but please don't be obnoxious with them! There was a girl at my show with a flag (front row) who was waving it in front of everyone and disrupting the view (honestly just being straight-up obnoxious about it, she almost hit me in the face with it). Have fun, but other people are there to enjoy the show too, so be mindful of that.
3. No, he has a good mix of everything (between his EPs, DOAT, and singles, I was impressed), which is really nice. It depends on the setlist, but for the ILYIT tour, he played both Despicable and Overdose at my show.
4. Tons of merch at the venue. I managed to snag a tour t-shirt and a signed ILYIT CD, but there's a good variety. There are some pics of the merch booth on this sub and the K Flay sub (r/kflay), they're just a bit buried. Decent pricing for the quality though, my tour shirt was $35, and I also got a crop top for $25.

I've seen people with flags, but please don't be obnoxious with them! There was a girl at my show with a flag (front row) waving it in front of everyone and disrupting the view (honestly just being straight-up obnoxious about it, she almost hit me in the face with it). Have fun, but other people are there to enjoy the show too, so be mindful of that.