Can I play multiple simultaneous instances of the game on the SAME Windows PC?

[Canadane]

[*]Create a new, SECONDARY Service Account within my existing Square Enix account. This should then allow me to log into the launcher once and then hopefully start 2 separate game instances and use the Datacentre function to switch between Service Accounts. This is the option that I believe is being alluded to by the first FAQ linq above.

Switching Data Centres does not switch service accounts. The two instances of the game would need to be logged into two separate service accounts and they would maintain that.
So outside the free trial, you would be paying for the game twice and paying for the monthly fees twice to accomplish your goal.

First time I’ve seen this asked. Is it allowed in ToS?

Multiboxing has been okay since XI.
While there's whatever limit they put on running the program to 2 on PC, there's no problem in paying for 2 games, and running them both.