Horses & saddles both change shape over time, so should be checked every few months for fit. I think his neck is a possible indication this is a problem that has been going on for a while tho. The problem with pushing & prodding is firstly that you may get a reaction from any horse, depending on how you do it & their training, & you may not get a reaction from a horse who is accustomed to pain & or desensitised & learned not to react to what people do to it. Sounds like he's of the latter & you would only get a reaction if there was acute pain near the surface in a particular area.
Treeless saddles can be great, but I would hesitate to put one on a horse with back probs, an unfit horse(get them strong first), under a heavy or not so great rider.... It's also generally not good to use stirrups for more than light balance, or mount from the ground. A crupper & breastplate is also advisable too.
Tried a variety of treeless. You can get treeless that are essentially the same as a traditional english saddle, so if this is what you're used to, this may be the most comfortable for you. I don't think they're uncomfortable generally - often more cushy - but you do tend to sit a little wider, as treed saddles are often made narrow for rider comfort(& discount horse comfort)