A couple of thoughts:
Minimum diameter is 0.75" for webasto
Fittings for PEX tubing are internal fittings and are less than the required 0.75" diameter
Looked into using PEX for a buddy's install and came back to the tried and true method of 3/4" copper for the long runs between heaters and heater hose for the connections.
I use a loop system for four installs I've done. Include a "summer" loop. This circuit involves the Webasto, water heater (if included) and surge tank.
I don't crossconnect the water systems, engine coolant and Webasto, because loss of one means loss of both. With my luck I'd lose an engine hose when the outside temperature was low. Then I'd have no motion and no heat and my bride would not be happy.
Just my $0.02
Bill