hello.
1.you can play i guess 8ths with heel toe. At slower speeds i noticed it tends to break down into two separate foot motions, and at higher speeds you'll only be able to pulse (that's why you don't see many people playing death / black with one foot). The really fast players out there, besides having fast bass single stroke rolls, do double stroke rolls, where each double is one heel-toe movement .
The biggest challange with heel-toe is not learning to play it, but learning to integrate it with the rest of your playing (switching from single strokes to heel-toe or putting those extra 32nd notes in your beats that make them soo sweet
).
4.Also, I think you should be able to do anything with heels-up that you can do with heels-down (i use this btw, just because it felt more natural when i first started).