Underground Propane line to fire pit

You should line it with firebrick or (since you like molding and casting things) castable refractory. Edit: on second thought, stick to hard firebrick - most castable does not like to get wet. Regular concrete is prone to spalling & cracking in direct fire exposure.

Depending on the shape of the "pit" a 2" layer of sand can work, if it will stay put (ie, if the "pit" is rather shallow)

I doubt enough heat will travel downward to damage the pavers, though you might consider a couple inches of sand below the lava rock as extra security.

Even if you crack a paver, is that a big deal? Keep a few extras on hand for when or if you move the heater.