I would like to skip category, subcategory and model selection when escalating interaction to RF. Is there a way to pre-evaluate those values?
Yes you can do it. When you define these fields on the $L.file before call the request.new the selection is not shown.
The extra work is create either the ifs aditional process to decide when it should appear and when it should not appear... you can use variables for that or different buttons (calling different processes).
I did a test on 9.6 OOB and it works like a charm. Just add the code bellow on the rm.open process, on last line of rad expressions just before the call of request.new rad.
category in $L.file="Generic Request";subcategory in $L.file="Hardware"
On OOB there are no models for this combination... just adapt it to a valid combination for your environment...