Is heat bending of pvc conduit allowed?
You need a junction box or other access every 360 degrees (or rather, no more than 360 degrees between junction boxes or access points.) It's to make it possible to actually pull the wire into the conduit, so embrace it. 

Things like an LB count as an access point and are frequently used to make this practical, but where needed, simply inserting a junction box in the run is easy to do - no need to cut wires or make a junction in it, you just need the access.

As for bending, yes, large radius sweeps can be purchased pre-made (real electrical supply .vs. home cheapo, etc.) and yes, you can use heat to bend PVC conduit - I've done it with boiling water, or you can rent the heat blanket type benders. I would not suggest direct flame for bending heat. You do need to take care not to kink the conduit when bending it.