OP, you are not alone.
From Ars Technica's Forza Horizon 4 review:
I've come to realize that this testing rig (i.e. MINE) will no longer load any installed UWP games. Same exact crash as you: splash screen appears in a window, then it vanishes with zero crash notice or explanation. I have other Windows 10 machines with my credentials that I have definitely installed UWP games/apps on, but I've never tried to run any simultaneously (certainly not in the past few months). I've even gone into my Win10 control panel to "confirm" that I'm really me, verifying my acct via an email 2fa dealie, and still no dice.
........we are definitely looking into this. And it's not the first time I've gone through this song and dance, going through many of the same steps you listed in the FP. I'm slightly heartened to see you list the exact same issue and confusion, so that our report won't sound like isolated ranting. Plz send word if you have any breakthroughs, and I'll try to do the same.
I'm very sorry to hear that! I will definitely share any updates I have on the situation and I appreciate you doing the same. Your situation sounds even worse than mine if not even a full reinstall could fix it.
I've never had a problem pausing and restarting a download in Steam, or heard of anyone else having any problems with Steam downloads. I've never had problems downloading from another client or heard of anyone having problems downloading from another client. Download issues seem to be a common theme with the Windos store. Im not sure how the company with 130,000 employees who makes the OS can't get this right.
Yeah even Nintendo in their very conservative online development culture never had any remotely similar big issues.
A little update on the situation:
Whenever I tried to launch FH4 I would again crash with DCOM 10010. I thought I could already play because a friend on XBOX already can without any region-messing but yeah. I found it strange that people on Era said they can't even hit the Play button on PC when I already could so that got me thinking and I had an "AHA!" moment. I logged back into my friends account (the one with the Ultimate Edition) on Microsoft.com and saw that my PC was still attached to his account which I promptly reverted. Now looking at the store I couldn't hit "Play" anymore and I got the message: "This product will be available 02.10.2018" So there is some progress, it seems my Windows still wanted to grab the license off my friends account. I'll give FH4 one more download and if that fails I'll do my last resort.
I do monthly image backups off my OS SSD, I will then reeimage my PC to the state of September 1st and then try again before I even started all this account-mess, if that doesn't work either I'll be at my wits end.
Update 2:
I changed my Windows region to NZ and restarted the store. All prices were shown in NZ $ and the "Available Soon" button was changed to "Install" So far this looks promising:
Thank god I have today and tomorrow off because of medical appointments which means I'll just wait three hours again and play Dragalia Lost in the meantime :P