The simplest means of bypassing the problems you’ve outlined are
1. Rest the floating element of the router on another piece of material matching the thickness of the section being routed. This could be an off-cut or another section of timber prepared to match.
2. Extend the base of the router plane by adding a wooden sole plate.
3. Threaded elements can be locked temporarily using thread-lock adhesive.
I’m not a Veritas router owner or user, but the above are potential fixes for the problems you’ve mentioned and I hope they help in some way.