I'm always a 'belt and suspenders' guy when it comes to wireless/network best practices. So here's my 2 cents and how-to...
1. You can provision your AP onto a specific antenna (see screenshot 'Enable-Single-Chain_Provision) via 'Configuration > AP Installation'
2. I would put a cap on the ANT1 port just to be safe and make sure that the unused antenna is loaded with 'something'. It can't hurt and can only help make sure it works correctly andprotects the radio should it be mis-configured later (it also makes it physically/visually obvious how it is configured).
3. Configure an AP-specific RADIO profile to disable CSD (see screenshot 'CSD-override')
4. Configure your HT profile on the SSID profiles assigned on those APs to MCS0-7 only (see screenshot 'mcs0-7').
Again, because you are splitting the RF into two different antennas covering two different areas, you will very likely see performance issues (your previous map didn't make clear to me what you were trying to show, but your drawings with the splitter show two separate antennas in two separate rooms, which will not work well).
As far as the steps, you're doing what you need to do, short of visually confirming that you have provisioned the radios and that the antennas are indeed connected to the correct antenna port (which I assume has already been done).
Edit: No not every single step above is required, if you are provisioning on to a single antenna, per your screenshot, and have CSD override enabled, that should be all you need to do. But like I said, I like being doubly/triplely sure so I recommend terminators and static MCS rate limiting.