I may not have made that clear in OP: But I can still access my wallet in mSIGNA.
Yeah, I get that. My point of importing the seed phrase again was to ensure your wallet is indeed just a single signature wallet derived from that seed phrase and not some more complex multi-sig set up which we would never be able to recover using just one seed phrase.
My guess it's only about finding the right derivation path.
Having read through the .pdf again, I'm not so sure:
NOTE: mSIGNA™ always uses pay-‐to-‐script-‐hash addresses, which begin with the character 3 in Bitcoin, even for 1 of 1 accounts.
There is no mention that they use nested segwit addresses, which is the type of address every other wallet will try to recover when given a seed phrase and a P2SH address type. The sentence above I've just quoted suggests they may use some strange custom 1-of-1 script, in which case you will never find it by just playing with the derivation path. You will need to find the exact script being used.
Does mSIGMA have the option to import transactions from elsewhere? That would be the easiest way forward - create an unsigned transaction elsewhere, import it to mSIGMA for signing, and then export it for broadcast. If not, then I think the easiest option is going to be to spin up your own node and get mSIGMA to connect to it. Otherwise you will need to comb through the entire source code to figure out exactly the script type and derivation path which is being used.
Alternatively - have you spend from any of these addresses in the past? If so, we can view the script for them on the blockchain.