You have made the assumption I would put the scissors jacks on the end of the frame, shame on you do not make assumptions.
You say the frame is not made for lifting, question what would you do if you had a flat tire.
You again assume the frame is not sufficiently strong to lift the trailer, like any thing else you should not assume, you plan and get it right. The frame holds the negative weight of the trailer, it should also hold the positive weight.
Consider people hang "stuff" from the bicycle rack, have you ever considered the weight of a couple of bikes, not just the dead weight of the bikes but the weight of the bikes on the end of a six foot arm for leverage.
The frame has to support that when you hit a bump and the trailer bounces, this could be equal to several times the actual weight of the bikes.
Yes the scissors jacks could be used to stabalize the trailer, and if properly placed could be used to lift the trailer, I find this concept more advantageous than carrying a supply of blocks of different dimensions to pull the trailer up on to level it, by trial and error.
From experience on our trips I have found only one or two camp sites that offered a level site.
On our last trip we stopped twice at the factory, (Reece & Tammy were away) Dave informed me that the scissors jack I am refering to are installed on the 5th wheel trailer, that if I wanted Reece could ship me a set, or for a fee have them installed.
The actual work of installing them is not a big deal
I wonder why this is not an option.
Doug