I don’t know if I’m misunderstanding the question, but it seems like it would be hard to recreate a source (in any detail) based only on a single (edited!) witness.

With Q, we have two witnesses, so one can compare them and make a reasonable guess about what the original contained, even when both witnesses reworked the Q material.

With Special M and Special L, you know what Matt and Luke record, so, if one records something nowhere else attested, you can surmise that each may have been borrowing material from a special source, but you have very little to go on in trying to figure out what their source recorded as opposed to any editorial changes each of them introduced to their respective source. E.g., how much of Luke’s infancy narrative did he just make up to suit his ends and how much did he borrow from a prior source? It’s hard to be sure since there is no parallel to compare it with.