I believe I have done it! (See yesterday's post.) I was struggling with one last issue where the PCs of the party would keep swapping back to the main PC during any conversation. Then I had a sudden thought to check the Campaign Editor and voila! I discovered what DLGPartySwap now does. :) It does exactly what I hoped for and when FALSE, it stops the dialogue returning focus to the main PC. Once this was set, it solved all my remaining problems that I have tested to date. Although, I am quietly confident that by doing this I have resolved all my issues.
And so, the bottom line is, I believe I have managed to allow players to add a party at the start (or not, as is their choice) and still have all my systems work OK. Just to reiterate what I had to do to get this to work, in a basic outline was this:
1) Swap the original created PCs for copies with new Tags. (Unspawn originals.)
2) Edit 2da script and attach companion type scripts to them using SetCreatureScriptsToSet. (See Script below.)
3) Set DLGPartySwap to FALSE in the Campaign Editor section.