You're going to be wrong about motive for purchasing most of the grandfathered multiple personal houses. Transferring housing items between characters on the same account is a royal pain unless you've got them all in the same FC with a FC house, in which case you've got the FC house for the storage and don't need the extra houses on your alts. It's not worth storing a housing items for future use when the majority are extremely cheap to make and you're going to have to do some character switching gymnastics to get to the item you want.
The multiple personal house owners have the extra houses on their alts because they wanted their alts to have houses to decorate, additional garden patch access, or to keep out neighbors who might decorate houses in way they didn't like. As others have pointed out, the number of players with multiple personal houses on the same world is very small. Most with multiple personal houses have them on different worlds, which is still allowed by the current rules.
Now there are a lot of players with single houses who are only using them for storage and have admitted such. They wanted a place to store the housing items they got through seasonal events, quest rewards, etc. in case they ever got interested in housing in the future, at which point they'd already have a house to decorate. Apartments can't store outdoor furnishings which left a house the only option. They had the gil, houses were available so they bought one. It's up to them how they use their house, not up to you to say whether or not they're using it in an acceptable manner.
Multiple FC house owners are a different creature. They're generally after multiple garden patches, workshops and wheel stands to make gil.
I suspect you'd be wrong with your guess about how difficult it would be to properly code this to prevent errors from happening.
Far more likely SE would have to demo every single house regardless of owner and force everyone to repurchase. That's not going to happen.
I understand the frustration the player base is experience with the current housing situation. But what you're asking for is yet another band-aid solution that would give a few dozen players access to a house without fixing the real problem, which is supply.
I'd rather see SE spend their time fixing the system than wasting time on changes that accomplish nothing in the long run.
Trading used housing items around is possible. Just need to share tenancy in another player's house or be in the same FC. I do this a lot, shifting items between my alts when I'm redecorating their apartments and FC rooms.