You'd have to tell the game that "you have the merge items", it's a value that can be changed but not by normal standards.
Inside of the main .as files, are certain codes that tell the game the player has the "items" necesary to "merge" the item or to take it. So a normal generic trainer won't be able to do that, and even if you're able to mess around with the game files; then the "modified game" would have to be on the AE servers to even work at all.
Of course, there's other ways but that's just one of them.
I'm not quite sure if there is a "Variable" that would change it, or if there is any ones at all that modify the game's behaviour like that; but there could be.
As for Shoplifting House Items, you'd have to have a DA in the first place; temporary ones only apply towards normal items, and won't work on Strict-Da items like house items. The game literally "knows" at all times, as to when you're trying to modify it. It's just that it's partially-poorly written (anti-cheat wise), which allows trainers to be made in the first place. So even if you were "able" to shoplift it, you'll still need to have the Dragon Amulet or it won't work at all.
And the whole "merge" shop really isn't a normal shop at all, the game detects constantly if you have certain items in your inventory.
These Items Contain:
And Item Names
THEN.. it detects if you purchased the item at all.
If one of them doesn't fit, you can't merge and get your item(s).