I'm far from being an expert on this, I just got my first pair (westerns) last year, but I figure it can't hurt to toss my experiences out there so you can get some more opinions.
1. I've never noticed!
They definitely don't feel smooth like some other species, they have texture, but I don't think it's anything extremely noticeable. Nothing like sandpaper that could cause discomfort when handling them. Try to feel some other snakes with keeled scales if you want an idea.
2. I want to say yes, but I'm honestly not sure about
Lystrophis. (And I've only ever read about
Leioheterodon doing it a few times) So someone else will have to answer that.
I will say that all
Heterodon seem to do it to some extent. I think certain species are more pronounced than others, personally I feel Easterns do it best... Also it depends on the individual! My male will huff, puff and give me the whole show, but my female is docile and has never done anything like it.
3. Malagasy Giants are the biggest and I think they get around 5 feet or so. I hear that the other two Madagascar species are also bigger than most. Pretty sure that the longest
Heterodon are Easterns, but honestly hogs aren't known for being big snakes and usually only get a couple feet long. If you're after a snake that gets 6-8' then I wouldn't think hoggies are what you should be looking for.
That's actually one of the reasons why I love my Westerns! The female will get much larger than the male (Makes his defensive displays that much more comical) but they'll always be small snakes.
4. My male needed some scenting at first, but now both my Westerns chow down on f/t rodents without a hitch. I think most species can be converted, but it seems like some may need more work than others. You should be able to find some for sale that are already good rodent-eaters or at least on scented.
5.
Pretty sure they're all harmless unless you're allergic and that none of them are real prone to biting, but I really don't know which would be worse.
Personally I'd be a bit more wary of
Leioheterodon species, but only because I'm assuming they can bite easier than one with a tinier mouth. I really haven't a clue though, so hopefully someone else has your answer.
Hope this helped some~