The USB connection is only used if you don't have a suitable RGB header on your MB. This allows you to control the LEDs with AMD's app instead of the RGB app for your MB.
That's still a strong build. The only thing I would do is upgrade your GPU to something like a RTX 2070 Super. Maybe upgrade your HDD to a 4 or 6 GB while your at at. 1GB isn't a lot for games.
First do a complete backup. Then just swap them to the new MB, enable RAID in BIOS, cross your fingers and boot. If it recognizes your existing RAID array, YAY! If not, create a new array and restore from backup.
This is the major problem. I've found one review for it on Newegg and it doesn't look good. Functional, but you'll be better off with spinning rust if the reviewer was correct about transfer speeds.
eBay is probably your best bet. Use PayPal and if you have issues you can file a claim for your money back. You can even spend a little more to get sealed OEM copies and you'll very likely not have to worry about it at all. Seems they can be had for less than $30.
Overclocking can have different affects on a card depending on use case. We've been seeing this a lot during our Folding @ Home event where people have very stable gaming overclocks but then start crashing or having issues when firing up compute tasks. Make sure you burn in your overclocks in as many usage scenarios as you think you'll use. Go back to stock and see if the issue still exists. If that solves it you'll need to re-evaluate your 'stable' overclock.
I try to avoid brand names that have little to no reputation. If you have to pay a little more to get something from a reputable manufacturer that is more likely to support its warranty you'll be better off in the long run.
Remember: Buy Once, Cry Once
Maybe cut some 1x2 lumber to fit front to back. Fasten with screws through the lip of the desk and a small bead of construction adhesive down the length. Then you can fasten the keyboard tray to the wood.
Quicksync uses Intel graphics. You'll likely have to enable it in BIOS and install the Intel Graphics driver. Change your encode settings from 720p Fast to 720p HQ. It will take much longer (maybe not with quicksync?) but you'll get a better encode.
Edit: I have an Intel CPU but have never used Quicksync because I don't have Intel graphics available. YMMV
No. You'll want to hit the custom option in the installer. From there you'll be able to resize the partition manually. I think you'll also have to define your / (root), /home (if you want it as a separate partition), and swap partitions (should be about twice your RAM). A separate /home partition is nice that way if you have to reinstall the OS you can do so while leaving all your personal files and config files intact.