What issues are there when hand loading .260 Rem when shooting in a short action vs long action with regards to seating depth and proximity to the lands?

With the added length of a LA magazine it would allow seating really long bullets out to the lands while retaining case capacity.

Yes, but the gunsmith suggests that with the longer magazine the rounds will be forced forward by the recoil and mess up my seating depth.

Nope; they won't; I use long actions for most everything; 6.5 creedmoor, 260 308, and others. They work fine. Recoil is the same and the bullets will bump against the front wall in a short action as well wouldn't they?. With the long action, the cartridges stay in place as the recoil does not seem to move them very much.
Latest build is a 350 Rem Mag on a long 700 action, which I had at the range today.. No, the rounds did not move forward enough to affect anything. If anything they don't hit the front wall at all in a long action but in a short one, they might.

Modern actions don't have the milled receiver integral shoulder stops like Mausers and Springfields do, but it doesn't matter.

You could add magazine shoulder ridges to keep the rounds aft if need be. Just like military mags.

Trust me; you don't need it; if I can put a 350 Rem Mag in a long action 700, which I did, and the rounds don't move, which they don't move much, then a 260 will work. Which I have done too.

I shoot a 260 and there's not enough recoil to move properly seated bullets.

Thanks!

Same stuff you can find on any interdnet gun forum. LA, SA? No mention in the OP if the action is Rem 700 or a Mossberg. I do know this, get that bullet tip stuck out there far enough and you can really be jumping through the snake pit getting the darned thing to feed as you would like. I have has some dummy rounds sent to me for throating that are so far out of proportion there is no way the shoulder will be on its way up the feed ramp before the bullet tip impacts the end of the barrel. There are just some thing things that are not going to work well together. But because "it was on the interdnet" it must be. Use a short action, have a Wyatts extended box install. Keep the length of your rounds within reason and you will have zero problems, as long as your gunsmith is more than just a "kit gun" assembler pretending to be a real gunsmith. Bullets don't have to touch the lands to shoot well.

Ok, I note that there are skeptics. True, this is the internet, but the difference is that I have done everything I say I have. And I have put all manner of short cartridges on 700 long actions. As I said, I had a 350 Rem Mag at the range yesterday, and I have a 6.5 Creedmoor in front of me right now. They both feed perfectly. So, theories about snakes are fine, but proven reality is better.

The action is a CZ Sporter Short Action. I've already ordered it through Bullberry Barrel Works. They are conveniently located between my house and civilization. The original is either a .243 or .308. Both have the same bolt face. They are going to put a 20.5" Wilson barrel onto it. Their ordering it will save me on mark up and transfer costs, and I can pick it up right from the shop when they are don.

I've never had set back in any rifle unless my reloading caused it. Back in the day I tried neck turning on hunting rifles without really knowing what I was doing. If you do happen to have bullet set-back it means your expander ball is probably oversize and could use some judicious sanding/polishing.