The switch to coarse threads, in regards to the Healey line, came about at the same time the BJ8 got different calipers, rotors, reduced diameter front stub-axles and smaller outer wheel bearings.
I guess someone decided the trade off was worth it__but were they an engineer or an accountant...?
I'm not aware of splined adapters cracking/breaking because of a too thin cross-section, only of the splines themselves wearing down, so the increased thickness__required for the coarse threads__seems unwarranted. But I've been wrong before...
If your front splines are fine-thread, it makes sense (to me at least) that the rears should be too. It's inconceivable that any of us would ever mix up a fine for a coarse threaded knock-off, but I'll bet someone, somewhere has done it, and what with having a BFH in their other hand, might've even gotten some traction with it!
I believe most Triumphs had four (4) lug rear axle flanges (not sure if the bolt-circle diameter was the same as a BN1 though) so I wouldn't go messing around swapping TR parts to the Healey. As for the wheels themselves, yes, the spline-drive hubs will interchange, as I used 72-spoke TR6 wheels on my drum-braked BN6 for many years.