Dual-Boot Install with FakeRAID+mdadm - Bootloader Error

I am attempting a fresh install of Ubuntu 16.04 along side Windows 10 and Windows 7 installations, on 2 hard disks under a RAID1 (FakeRAID) configuration. I tried to do some up-front research, because last time I tried this, I broke my FakeRAID configuration, having to rebuild my array. So, I found the following resource, using Ubuntu 14.04:

http://askubuntu.com/questions/45551...ror-question-m

Articles I read seemed to confirm mdadm was recommended over dmraid. So, I proceeded to deactivate dmraid and installed mdadm per instructions, selecting /dev/md126p6 as my bootloader device* after creating my partitions manually. After starting the install getting past the current appstreamcli issue (killing the process), I get the "Bootloader install failed". Per the instructions at the link above, I expected this to occur, and planned to revisit this later with the repair utility. However, the "Bootloader install failed" screen refuses to let me press "OK" - it is not grayed out. The window just seems frozen.

I have found several bug reports on this, dating back 5+ years, with a resolution marked "Invalid", but with a lack of seemingly good advice. Any suggestions are appreciated. Thanks!

*Output of fdisk -l for /dev/md126:

Disk /dev/md126: 698.7 GiB, 750153367552 bytes, 1465143296 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disklabel type: dos
Disk identifier: 0x197928fb
Device Boot Start End Sectors Size Id Type
/dev/md126p1 * 2048 206847 204800 100M 7 HPFS/NTFS/exFAT
/dev/md126p2 206848 503523327 503316480 240G 7 HPFS/NTFS/exFAT
/dev/md126p3 503523328 1006839807 503316480 240G 7 HPFS/NTFS/exFAT
/dev/md126p4 1006841854 1465141247 458299394 218.5G 5 Extended
/dev/md126p5 1006841856 1010839551 3997696 1.9G 82 Linux swap / Solaris
/dev/md126p6 1010841600 1465141247 454299648 216.6G 83 Linux