For a home office network with 4 devices connected via Wi-Fi all trying to stream YouTube videos on a 15mb down connection, which MikroTik Wi-Fi access point should I go with? What is the best MikroTik unit? Is mimo necessary?

The access point will be connected to a RB2011UAS router in a single level 2800 sq. foot home.

Should I get the RB2011UAS-2HnD-IN or the RB951G-2HnD?

Get something like the Asus RT-N16. Way, way more reliable and not that expensive ($84 at amazon).

And before you have a chance: Yes you can run it in straight AP-only mode.

Plus more range with the FACTORY omni antennas on the RT-N16 than even an SXT pointed down the same direction.

Heck you'll even find that it can pass as much, if not more, data in router+AP mode than your RB.