Those are good questions I think we're going to have to find the answers to the old fashioned way. The issue with the Ti is that O, N etc don't absorb so much as combine into very stable compounds (see how hard it is to reduce in the rare metals handbook - it's why it's so expensive). My
guess would be that you could sputter or blast off the bad surface layer if you put it on a little thick to start. Just have to try it!
Most of the older stuff used Zr, which is why it's in the literature. Zr lets go of D easier, so Ti would be better if you couldn't actively cool the target. Zr also likes to combine with air...
In the U of Wis paper they did something I think stupid - they evap'd the stuff with D present, which makes TiH - which is a very crappy material (stiff and brittle) for a thick coating, no wonder they had troubles. You can load it up the right amount after deposition by heating it in D, or driving it in with the beam, though, and that leaves a ductile matrix, should work better (and that's how the big boys report doing it). For T, you'd want to use the beam method so as not to have as much of that around the place. Evidently around 4-500C Ti will eat D just fine, you let it cool in that atmosphere and it loads up fine, wastes a little D that's left over, but not bad considering.
Ti has less stopping power than Zr, being lighter, so it lets any incoming ions that miss on the first layer retain more energy for possible fusion deeper in.
Clean metals aren't that low in secondary emission compared to say, carbon, though. No matter the metal, though, the more electropostive ones are the worst for that - and Ti, stripped of it's oxide coating (it gets one like Al) is pretty positive. Low sputtering is true of Ti, at least. But it definitely sputters some. You can see it on my tank walls from running Ti grids, and it may account for my somewhat better results?
Obviously we have more to learn here. Right now, I'm still chasing leaks, and what turned out to be a bad valve in my forepump, sigh. Gotta tear it down and rebuild it, and they stuffed it in there real tightly.
Posting as just me, not as the forum owner. Everything I say is "in my opinion" and YMMV -- which should go for everyone without saying.