1) I'll have to file out a groove on the outside of the plastic mount, the N connector jack and plug will not pass.
Yes, thats often to do.
2) The bottom of the reflector touches the metal arm, is that a problem?
The contact itself is no problem, but, if the distance of the focus is a bit wider from dish you have no chance to adjust.
At some offset system it is necessyry to enlarge the feed arm a few cm to get the focus point on the right position in front of the waveguide from the POTY
So, for excact adjustment you need first the electrical lense, because the focuspoint with lense is 1-2 cm in front of the waveguide.
Any other options for a good lens?
Do you have someone who can print 3-D ?
You can 3-D print a electrical lense.
Or turn a piece of Teflon with a lathe.