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About to go to bed again but I'll answer the questions! I do agree with your plan for 4 30 minute sessions, though, that is much more practical :)
1. If you've ever imagined talking with a friend in your head, and you imagined what they might say, it is similar in category to that, but significantly more distinct. It feels like another "self-voice" in your head, in the same way you talk, but "coming from a different spot." Understandably, the language for this kind of stuff is very imprecise--but an analogy to a known value, like imagining an argument in the shower, that kind of thing, is an effective way to communicate what it's like.
2. That comes from consistency, and spreading forcing out through the day, so you're already doing part of it! I am very able to interrupt/speak up without prompting when blank is consistently forcing throughout the day in small sessions-- it comes and goes depending almost entirely on that, + sleep/stress levels. Another thing that helps is "passive forcing," which is just doing what you do in your current 2-hour/4x30 minute sessions, but while you're doing something else. "I'm gonna play this video game, but narrate what I'm thinking about to my tulpa," "I'm gonna do work, but talk about what I do and what I like/dislike here with my tulpa," that kind of thing.