I don't know the answer to your first question, but I probably would not buy one, should one exist. (There are programmable keyboards, of course.)

A quick glance at the unicode provision for mathematical symbols (e.g., here) shows that there are too many of them to fit on a standard keyboard with 100-odd keys, even with modifiers.

This is similar, though not perhaps as severe, to the problem of trying to write in Japanese using either kanji or kana on a keyboard. (Same problem in Chinese, of course -- it's just that I've more familiarity with Japanese.) The solution there is not to have a huge keyboard with lots of symbols, but for the software to do the work. All one would need is an input system for mathematics. For me, although I concede it is not optimal, this is TeX. (In fact, TeX is recognised as an input system in Emacs.)