I agree that person-ness is cluster of properties and not a binary.

I don't believe that tulpas possess a significant subset those properties independent of the person whose tulpa they are.

I don't think I'm failing to understand any of what's discussed in Diseased Thinking. If there's something in particular you think I'm failing to understand, I'd appreciate you pointing it out.

It's possible that talking to accounts that claim to be tulpas would change my mind, as you suggest. It's also possible that talking to bodies that claim to channel spirit-beings or past lives would change my mind about the existence of spirit-beings or reincarnation. Many other people have been convinced by such experiences, and I have no especially justified reason to believe that I'm relevantly different from them.

Of course, that doesn't mean that reincarnation happens, nor that spirit-beings exist who can be channeled, or that tulpas possess a significant subset of the properties which constitute person-ness independent of the person whose tulpa they are.

A newborn infant or alzheimer's patient is not an independent individual distinct from it's caretaker either.

Eh?

I can take a newborn infant away from its caretaker and hand it to a different caretaker... or to no caretaker at all... or to several caretakers. I would say it remains the same newborn infant. The caretaker can die, and the newborn infant continues to live; and vice-versa.

That seems to me sufficient justification (not necessary, but sufficient) to call it an independent individual.

Why do you say it isn't?

Do you count their destruction as property crime as well?

I count it as less like a property crime than destroying a tulpa, a novel, or an RPG character. There are things I count it as more like a property crime than.