How can alexis like being set on fire when it can't be set on fire as it has no physical form?
Easy, simply light her on fire mentally.
Doesn't require physical form at all.
The contradiction lies in you explaining that tulpa's concept of a corporeal experience is based on your own but then going on to say that it isn't.
Simple, she has the framework on how to act from input, she just has to change it.
Edit: Screwtape, i have this to say