I would probably use their flat mount dropouts if I was going that way with the components. I was just looking at flat mount, and apparently if you want to use 160mm on the rear you always have to use an adapter. I have seen people say that's a limitation of the polydrops (that's what I thought until just now), but it's not true. I have some frames planned with 10mm dropouts, but that's because I have the dropouts and I'm not going to let over $200 go to waste. I think 10mm 135mm hubs will be around for a while, hub manufacturers probably are going to continue making hubs modular into the future because they have been burned so badly by fast moving "standards" changes.

Another thing that might determine this is dummy axles. I have both 135 and 142 dummy axles. If I was going to buy, I would go 142 and just skip 135 altogether. It's not going to be getting any more popular.

As far as mitering goes, I didn't notice who started this thread, carry on. If I had a mitering system I felt confident in, I would do it that way, just cut the small diameter at the right angle at the right location and open up the miter from there. I have to try my lathe again now that I fixed the cross slide lead screw. It used to have enough backlash that I would break saw blades.

Not that long ago, I saw a post from a framebuilder who had long, sad experience with integral headsets and recommended the plain version. Granted, that just reinforced my existing bias.