I am very pleased for you. I've lost track but I don't think there was anything hugely wrong found so far in sorting out the coolant loss? That's great.
I guess, and I can only guess, that either the problem developed after the previous owner sold the car, or, perhaps they knew there was a niggling coolant loss and couldn't trace it. So far as heater hoses are concerned, they are often less than easily visible or accessible on some engines. If so you've done the graft and can reap the benefits. And we'll never know how competently the pressure testing was done.
Saving in case of a serious underlying problem sounds like a plan. There have been hints of people worrying that there might be something serious. Possibly something addressed by that additive you used.
Best case scenario you end up with a good ongoing car, and some savings. Cushti!