@SheriGRThanks for your input and effort. This solution will actually work in some cases, no doubt. They still have some major flaws:
1. Saving household will preserve major relationships within said household, but, as you've mentioned, it will destroy all progress of current sims. In most cases, the sims that need to be restored are/were a part of some bigger major player's family - that's the main reason WHY players want to restore them. Sacrificing other family members progress is not an option..
2. Creating relationships through cheats or mods is the same workaround I've described in my OP. It works, but again, it has its flaws. In most cases, it will not restore family tree properly, leaving all the question marks in it.
So, just as you've said, an imperfect and spotty solution. But the sole existence of said solutions proves it's doable, even if third party mods should be involved (like SimPE in Sims 2). As mod developers explained to me, at the moment it is beyond their capabilities to create something that would fully restore a culled sim. Only the game developers can expand on this.