Moreover, the Lost Kingdom interpreter will expand the memory arbitrarily as it encounters > instructions, whereas the bf2wasm interpreter has a fixed limit of 16384 4-byte cells according to its documentation.
Oh, you are right; I simply overlooked that part of the readme. In my opinion that should have been 8-bit cells mainly because AFAIK simulating 16-bit or 32-bit in 8-bit cells is reasonable [1] but doing the other way without slowing it down too much is highly dependent to the program.
Signedness depends on your interpreter. In practice it hardly matters since there's no print function anyways, so if you want 2s complement you'll end up implementing it yourself anyways.
[+] should always overflow to 0 at some point, which is entirely doable for 8 bit, but less so for 32bit. as for choosing between [-] and [+] you'll just have to guesstimate if 0 or intmax is on average closer to your value.
[1] https://esolangs.org/wiki/Brainfuck_bitwidth_conversions