Thanks for responding. A few days after posting my question, I gave up and filed a Quebec return on paper. I had to find the right location in the Quebec government website, download the (non-editable) form, print it, fill it in by hand, scan a copy for myself, and mail the original to the Quebec revenue department, along with a cheque (!) for the amount due.
The root problem is an unnecessary restriction in the software design: unless one fills in "Quebec" as the province of residence, it is impossible to access the Quebec forms (e.g. TP1).
Most residents of other provinces do not need the Quebec forms, but there is no reason to make them totally unavailable. There are real cases, such as mine, which require Quebec forms, even though I live in another province.
I hope the developers can remove this counterproductive restriction in a future version of the software; even if it only provides the ability to fill in and print the Quebec forms, that would be a huge improvement.
