The hippos have never been contained at Longleat, and have shared the lake with the sea lions since the 60s. Longleat last bred hippo in the 90s, and there were 3 distinguishable sets of calves that were born having effectively free-roam (admittedly the mortality rate was high, but times have changed).
The fact they're building the new house on the site of the old one tells me it'll be a similar if not same process to the previous hippos. Yes, bulls can be grumpy, but they can tolerate crocodiles in the wild which are somewhat more threatening than a couple of nosey sea lions.
How is the gene pool in the US looking? Personally I don't see that happening if just for the costs and difficulties involved as you mention, but I can think of worse solutions. Fritz surely can't be much different in age to Hodor right? All I know about that pod in America is that they are struggling for space