Yes if the tub are sitting directly on the heat tape over time it will rub and cause damage, which means you will have to keep a close eye on that rack.

As for heat tape 4 inches is pretty standard when it comes to PVC rack and more than enough, if it wasn't no one would buy those racks.

All my V70 have 4 inches in the back so yes it is working just fine.

When people recommend to have heat covering 1/3 of the enclosure it's for less efficient enclosure such as tank where there is a massive heat loss.

If your room is not below 75 a 4 inch hot spot will work fine and 2 more inches will not offer much help when it comes to the cool side as this need to be regulated based on the overall ambient temp of the room or a secondary heat source.