What screws to use with Underware system?

Hi folks. I'm starting to play with the Underware system after running into the explainer video from Hands On Katie, and I noticed that she screws the MB tile directly to the board through the small (diamond?) holes, rather than use the countersunk spot wit we get with the usual DS offset snaps.

1. What kind of screws work best for something like this?

2. I've been using #8 (4.2mm) screws, and the screw head holds against the angled back of the DS snaps. With the diamond holes directly, there's no angled bottom. It's a straight hole all the way through. How does any screw find leverage to hold that in place?

I hope my questions are clear. Thanks so much for your suggestions!