Personally, I find that totally unacceptable.
If you have a valid ticket, then you should not have to go through any of that at all.
The OP asked for practical advice, not for people to state the obvious. We all know that valid tickets should be accepted with minimal fuss, but it's not the case and that's the reality which needs to be discussed here.
I've had these experiences before. Both ended up with the guard "letting me off" because I had Priv discounted tickets. When it gets to that point, I say nothing other than "thank you." I'll allow them to feel like they've done a good deed for the day. No point entering into further discussion if you're exploiting some obscure rule that less that 0.1% of the travelling public will ever make use of. I know that some enthusiasts get some sort of pleasure out of "educating" staff who have the wrong attitude but this is counter-productive. Once they "let you off," let them go. Don't even say "but I was in the right anyway."
One such occasion was on SWT when I was using a London - Southampton ticket to travel via Salisbury where my ticket was checked after Basingstoke. I ended up having to use the routeing guide but that fell on deaf ears. Eventually, the guard gave up and said "I'll let you off this time, but only because you're on Priv."
On another occasion on EMT, I was using a valid combination of tickets, which the guard took exception to. After being patronised and condescended by the guard who was insistent that his 20 years of experience trumped whatever knowledge I had, I wound him up to the maximum possible extent by being extra polite and justifying my validity by showing the NRCoC. My upbeat attitude perplexed him as he said I was a troublemaker. It looked like he was going to pop a few vessels but that wasn't my fault. Eventually, he said that he was being more than fair by "letting me off," but the next time, he'd report me for intentional fare evasion and withdraw my Priv. After the train's first stop almost 100 miles later, he checked my tickets again and I revealed that I had yet another ticket, from the stop we'd just called at. He said "that's much better!" as if he was talking to a toddler. I smiled and let him carry on. After all, I use such tickets purely to save money. Arguments with guards is an unfortunate by-product which occurs on 9/10 occasions.
Seeing as I no longer have a Priv or Season ticket, will have to knock this creative ticketing on the head. My advice is to stand your ground firmly but politely. In my experience, it yields a positive result, even if not in the manner you would have preferred. If they want to MG11/TIR you, let them. It's only if you see them call the police to get you removed that you should agree to whatever sanction they're imposing. And that's only if you don't have the time to wait for the next train. You must be 100% certain that the ticket(s) you hold are valid though, otherwise you'd be digging your own grave.