I'm not an expert, but I regularly use three De-Vere enlargers. So in response to your questions:
1. Yes, I thought so.
2. Glass leads to better overall sharpness, but there's more spotting to do. I use glass.
3. They are used without metal masks.
4. I don't know anyting about Viponel timers. De-Vere timers are simple and work very well. (I have two spare ones.) The best timer, imho, is an RH Designs Stop Clock Pro, which is what I use with my De-Veres.
Peter De Smidt