If the toilet is designed to use pressurized water, no problem
My question is regarding contamination of the water supply tank from the head backflushing. I am unclear on whether one needs to put in a check valve or what.
But, if the toilet is designed to use sea water, it should never be connected to the fresh water supply--with or without any check valve. It cannot be done without risk of e-coli contamination of the potable water suppy, damage to the toilet pump or both...and every toilet mfr specifically warns against it in their installations
If you want to use fresh water to flush a sea water toilet, there are only two safe ways to do it: a separate flush water tank for toilet that cannot share any plumbing including fill or vent the the fresh water system or connection to a sink drain. You'll find a lot of discussion about how to do this in the HM forum.
Some people use a cup or shower head to just put water in the bowl...but that's not a good plan because it damages the toilet pump.
There shouldn't be any back flushing, so that's the first issue you need to deal with.
