I have done some work on the internal reflection issue before, but no testing since then. Today I have a cloudy afternoon and thus it is a good chance to test it out with a EOS 1.25"adapter.
First test was conducted with a diagonal. Yes, it won't focus unless I add a barlow in front of the diagonal or a 2x extender, 2x extender seems more elegant. In short, it's sharp with my 20mm eyepiece. I expect around 50x with this configuration.
Pulling the diagonal off should be a better test, it's even sharper since the effective focal length could go down to 300mm! I took the plunge to use it at 3000mm, yes, 3000mm by using a 2x extender plus a 5x Powermate, and so we have 150x and it remains very sharp and contrasty, too.
I guess I should try it out with the moon or the planets, probably with a camera. I hope it could replace my C5 eventually, if it is also good as a 4" refractor.
After writing the above short report, I pulled it out again to try higher.
First test was with 2x extender with a TMB 4mm, so we have 150x again. Sharp and no image break down.
And then I added a 2x barlows in the same setup, so we have 300x! Floaters come in but again no image break down, focusing was easy.
I'm quite confident now.