I realized recently that my segwit implementation was not correct,
basically some time ago, wrongly reading the specs (and misleaded by
what follows), I thought that scriptsig would go into witness data as it
was, but that's not the case, op_pushdata is replaced by varlen
Now reading correctly the specs, they seem to be not totally correct,
then the first question is: why OP_0 is 00 in witness data and not 0100?
Does this apply to other op_codes? This does not look logical at all
op_pushdata allows for unsigned integers and thus it uses the extra byte to specify that.
A varint (varlen) is unsigned. 0 and anything <= 252 is one byte length, so 00.
The second question is: why for non segwit inputs there is a 00 length
in segwit data, what is the rational for that? It should just be nothing
since you don't need this to reconciliate things
Because you don't have the number of witnesses as you have for inputs/outputs. The witness data length is assumed to be the same as the inputs length. And then for non-segwit inputs you need the 00 to specify that it is empty.