Get message "One or more devices are unreachable" after configuring a High Availability pair

"I realise I may need to reinstall the OVF templates from scratch (and request reactivation of the two licenses), but could someone kindly confirm the above explanation is a possible cause of the problem first?"

I can't speak to your entire issue, but for the above I can say I have had to redeploy a 11.4.1 VE in the past because I modified the network adapters incorrectly prior to bringing it up for the first time. I think as long as vnic 2-4 are different than vnic 1 you're all set. There may be more to it than that, but I've had a similar problem.