Another vote for bitless, at least until your horse can become more comfortable & responsive. I would NOT attempt to force him, with a 'harsher' bit. Regular snaffles are plenty harsh when used strongly.
Now I can ride him around with very light contact on this bridle just fine, he will do nothing more than stick his nose out. Because he is so fond of sticking his nose out rather than bringing it down and softening he has an EXTREMELY over developed muscle under the neck and almost nothing on his top line, his ewe-neck is pretty ugly. ...Picking up the contact with him and sponging and asking him to soften and go on the bit for me usually turns into him getting annoyed and sticking his nose out more or shortening his neck.
First & foremost, sounds like he's dealing with a physical issue there & is hurting. I'd want to treat/rule that out before treating it as a training issue. It is not a comfortable way for a horse to go, but it may have become the most comfortable position for him to carry you in, if he has saddle fit, back, neck, mouth, bit, rider balance, hoof, etc... issues. Have you tried him bitless, to rule out mouth pain at least? I would quit riding him until you've sorted the physical issues out.
Training-wise, it sounds like perhaps it's your mother that needs more training, perhaps to begin with on a 'softer' horse, so she can learn & enjoy. If he is fine for you to ride softly, I imagine it is your mother's nervousness that is keeping her 'tight' - through the body & hands, which will understandably make him more antsy & resistant. Using a 'bigger bit' to allow her to inflict more pain on him is only likely to escalate his resistance.
I would work on getting your mum more confident & therefore relaxed. Is she nervous about everything, or is she OK with some? If she's OK with some things, I'd start with them, or else just start out getting her to sit on the horse, &/or be led around at a walk. Only once she's confident & relaxed about this would I ask her to try *a little* more. Get her confident at each step of the way before *gradually* doing more, so that she won't tighten up & make your horse reactive & bracey.
By strong I mean holding him so hard on the outside that my inner elbow hurts
If it hurts you, imagine how it feels via a bit of metal in his sensitive mouth!!
I appreciate you've paid that 'expert' to advise you as such, but IMO it's people like that that need to be bitted & yanked around. He must be a wonderful personality, to put up with this without (more) argument. Sounds like your instructors have not helped you see things from the horse's point of view & have never considered that maybe he physically *can't* softly do as you ask. Perhaps (as well as fixing physical issues first) you need a more considerate trainer.