Yes, you're right concerning the fresh water pressure and the check valve.
It does remain that the toilet pump itself (the one in the toilet bowl base)
will not be able to draw fresh water from your tank. It is only designed to
move the water that is already in the bowl while also macerating.
To get fresh water into your bowl you will need at some point in the fresh
water line a pump controlled by the red flush button. Here your choice is
either to use the existing seawater pump and tee it, but piping becomes a bit
complex, or install a completely independent pump. If you go for the second
solution you will also need an electric switchover from one (freshwater) to
the other (saltwater) pump in order to avoid running both together.
One possible solution, if using the original seawater pump, would be to move
it behind the toilet bowl and on the right, but, judging from your hull
number, you probably have the airconditionning compressor assembly installed
there which does not leave much room for extra equipment.
All the best.