So, I'm back after I did the merge, and brought a few facts:
Lifetime Happiness points are preserved while saving to bin, that's what I knew, but I was surprised to see them to be still there after the household was managed by deault EA story progression.
Murphy's law works perfectly. Sim I needed to interact with was moving around the town in a way that made it a pain to reach him. Not to mention that when I loaded the save that I wanted to place to bin, I was burgled - which delayed me a bit...
Unicorn the family tamed remained there - that was quite surprise for me. So now there's a NPC household with unicorn.
Default EA story progression still doesn't care about days remaining to age up, and even though the teens (did not waited all the way to YA, I wanted to get them know each other while in high school) were 10 days apart (not much on my 320 days lifespan), she aged up when the boy was 35 days to YA - roughly 20 days than if I played her manually. Probably the most annoying fact that I really hope to be solved in Sims4.
In the end, I did not actually merge 2 saves into one (as the longer one was pretty buggy). I just placed selected members of both families into the bin (move out, place to bin, exit w/no saving), started new game, used one as the family I wanted to play and then placed the other from bin as NPC family when the time was right. I hope that'll remove glitches accumulated by 5 generations of save1 - seems to be successful so far.