Oh, I've got so much stuff for the Betelgeusians in general, but I'll just be specific to Ford right now, since he's a bit of a handful even on his own.

a)

Ford started hitchhiking when he was 12 (which seems rather young, but then there's all the muck about Betelgeusians in general that sort of makes it still pretty young, but not like how one wants to imagine him being young), and landed on Earth when he was 15 (which doesn't seem like much of a gap between, but then there's world building muck that sort of makes this actually a bit longer than one wants to imagine). And the reason why he left Arthur on prehistoric Earth was indeed because he was bored, which is quite fatal to Betelgeusians if left unchecked (but in a brief moment of tact, he refrained from mentioning to your Arthur that Arthur was, quite literally, killing him).

b)

Because he totally knows where his towel is. He has his own rules and his own way of looking at the Universe, and neither hell nor high water is going to change his opinion on any of this. Sure, he's a total loose canon and quite possibly dangerous, but at least he knows what he believes.

c)

There are so many things that I plan on exploring with him, both through fic and RP, most of which involves the sorts of things that happened to him before meeting Arthur, and even before landing on Earth. But the combined effect of all of this has pretty much turned him into an unruly teenager who happens to know how to hotwire spaceships and hack computers.

Specifically, I want to look at some of the time he spent on Gretchentown, New Betel (in my headcanon, New Betel is a star just north of Orion's Belt, on the inner rim of Barnard's Loop (HD 38529; have I mentioned yet that I take fandom way too seriously sometimes?), and Gretchentown is the fourth planet in its system), which is where he learned to do most of the more nefarious skills he has now.

d)

No one specifically. He does have a canonical predilection toward the female sex (although I'm not sure if that's Ford, or just a holdover from the source material being from the 70s), so I typically picture him having interest in women, usually in general. Being a Galactic traveller, I don't really think race/species bothers him too much, except for possibly in certain extreme circumstances. Eventually, I do plan on having him run into a Betelgeusian he used to date when he was a student, but I'm not sure what, if anything, I'll actually do with her.