Insert sigs in scriptSig in order of the signers (updated)

We don't know in advance what position we'll end up in.

Also, the existing code sometimes inserted in the third position for 2-of-3, which doesn't exist.
is this going to work when local signature is not the first in redeem script?

Imagine, we have 2-of-3 with keys k1, k2, k3 (sorted), where k3 is a local key and corresponding signatures are s1, s2, s3. We're going to sign with k1 and k3.
LocalTransactionSigner will insert s3 in position 0 and call to `getSigInsertionIndex` in Custom signer will yield -1 then (myIndex = 0 and the only sig's index is 2), which, I guess, isn't expected.