Including Linea Maestro: if they are "goodyear welted" as described, then that's a mechanical process, and involves gemming. "Hand welted goodyear" is a contradiction in terms, that I can only take to mean "using your hands to operate the goodyear machine!". I suspect that the Linea maestro are welted in house, rather than in China like the regular line. The only actual hand stitching their site shows in its video collection is for Norvegese. Which is presumably why it is another 100 Euros. I think if they actually hand-stitched the welts and/or soles, they'd make more of a selling point out of it considering what else they show on the site.
Again, I'm not having a dig at Meermin here. Goodyear welting on a machine is still a skillful process and John Lobb, Edward Green and G&G do exactly the same - and gemming too. But phrases like "hand made", "hand welted", "hand grade" and the like are deliberately vague. There are very few (usually bespoke) makers who actually sew the welt and sole on by hand, and I'm pretty sure Meermin isn't one of them. I think the real difference is quality control, like you've said: the more they do in their own factory instead of China, the more consistent they can be. Along with different leather grades, I'm sure that is what you pay the extra for - not just the painted soles.
Just to set things straight: Linea Maestro are indeed hand welted, exactly like for example a few other RTW-makers like Vass or Paolo Scafora, and pretty much all bespoke makers. That to me is the major thing with Meermin. But the hand welting takes place in China, hence the very low price for hand welted shoes. Just the "finishing" of the shoes that's made in Spain, this includes bot Classic, LM and MTO shoes. Why they call it Hand welted Goodyear is probably cause most people take the Goodyear name as a quality stamp, not realising that hand welting is actually better when you don't have to deal with gemming.
But I have to say that I completely agree that Meermin have to take care of their issues with long delivery times, unanswered e-mails and quality control issues ASAP. It could be a big problem if customers give up and don't stand the trouble and effort a Meermin order is currently often associated with.
Edit: Missed that Rdiaz har already answered about the hand welting.