Hi I have a 12hh Dartmoor who has no wither and I have an Ideal WHP which is fantastic even when she gets a little round!. It doesn't roll or slip atall. Hope this helps.
Thorowgood cob saddle, the 4th breeching girth strap is very useful for keeping the saddle vaguely in place! and can swap the inserts if they lose weight etc. Cheapish to buy and easy to resell as well if it isnt needed anymore too, making it less of a throw of the dice than a lot of leather saddles. And you dont have to keep it out the rain! I think they have leather girth straps now, try and get that one as the synthetic girth straps are awful, hard to do up and they crack, tho it only cost me £10 each to get them replaced with leather anyway so not a disaster.
I have a 14.2hh welshie x who also has an Ideal saddle - no idea what type, assume GP. He's had it years though. Got a Kent and Masters GP for my horse though (he has high withers though) but they do a cob and wide cob and they have adjustable gullets on them. They are essentially Thorowgood saddles but they are leather, not synthetic.
I specialise in fitting round natives and cobs and it's not an easy thing as you've clearly realised . No withers at all generally means at least a 6 fit, an XXW. You need a wide head to the tree, flat tree from front to back, flat panels (ie not sausage shaped), and four girth straps. I fit Native Pony saddles. Others that might work include the H&C tree from Ideal and the Haflinger tree from GFS.
i had a problem fitting my welsh D, he had the widest spine id ever seen on a horse and an xxw fitting ,i was very lucky i have a saddle fitter as my YO so lots of help ,i eventually managed to find a leather saddle on ebay after trying 8 in total so very hard work!