When was the last time that Holyhead had a full slate of spare turns for all essential staff to ensure that the first train to London almost always runs even if somebody unfortunately goes off sick? (I would presume that this means driver, train manager, (?) shunter, fitter, (?) station staff to open up/despatch; and, for that matter, a relief signaller/crossing keeper on Anglesea while we're at it.)
No idea what spare cover in the Avanti messroom is like, but they can't even cover the service (even after it has been cut back in the LTP timetable) at the moment so probably little cover, particularly with drivers. In an integrated railway of course the staff would be pooled so TfW would be able to keep things going on Avanti's behalf (assuming similar arrangements as when FNW drivers drove trains for Virgin) as they are much better resourced.
Holyhead can operate without a fitter, even if it means that fueling or minor repairs can't be done. In a crisis the Llandudno Junction fitter will have to drive down.
In the event of there being no station staff at Holyhead, TfW services still run, with the guards being instructed to self-dispatch. I don't know what Avanti's rules are (I wouldn't fancy self-dispatching a 10-car Voyager from platform 2, though a five car Voyager from platform 1 should be fine). Avanti already self-dispatch at other stations in North Wales (some of which have curved platforms) when there are no station staff on duty.
I can't say that I recall a closed signal box on the coast. Possibly Ty Croes crossing (with the gates locked across the road, as they are at Llanfairpwll every night). Penmaenmawr can be switched out anyway. It's not like Hereford which went through a bad patch a year or two ago.
A look at Recent Train Times paints a picture. One Avanti working gets cancelled more often than it runs, with another almost as bad. Looking at the TfW services I can only see one instance of a train being cancelled due to a lack of a driver (1V90, 27/06), otherwise unit faults and infrastructure issues predominate. Some of those cancellations were late-running trains being turned back at Bangor, so the majority of passengers weren't affected anyway.