The two mounting holes on the TT for a TT bag are definitely there. I will answer the rest of the questions with the geo. But please keep in mind xmcs are designing an aero (race) gravel bike and nót an adventure bike, daily commuter etc etc.
My philosophy is that you need to stay focused on what you are making. If you make a bike that ‘has it all’, it kind of loses its identity and will do many things good, but nothing really good.
Well it all depends on what size we are looking at.. me who usually rides medium/small (177cm long) - then around 1.45-1.46 is quite normal, while larger sizes tend to run lower - in range of 1.42 or so, and smaller bikes xs can have higher STR.
The problem with a too long frame is that it is always easy putting a longer stem by 10-20mm, but when you are down to 60mm stem, there is nothing smaller avaliable (half integrated) - and integrated bars is usually not avaliable below 80mm.
So rather make it a little shorter than making the frame to long...
By the way - what is missed in STR is still also seat tube angle, so i usually rather take stack to top-tube ratio, and there you have a better measurement.
I my case my bike today does have 551mm top tube to 574mm stack. Ideally i would have approx 5mm shorter top tube but same stack.