1) how common are comms failures? I mean actual failures not just the muppet on board has dialed the volume down ( yes I've done it!), given that I have two radios on board, should a backup hand held be a priority?
It all depends. There are planes with frequent ghosts in the machine and there are ones which never get failure in COM, but - you can not see it from the outside. I always gave a handheld backup priority by a single reason - if I am getting into trouble I want somebody to talk to and help me. Even in a three COM plane you will have a need if you have to power off ...
2) If I do get one, what features am I looking for. I've browsed a bit online and all I can see difference-wise is that some are more expensive than others. What are the must haves? Nice to haves? avoid at all costs?
I would go for simple COM, no need for VOR, ILS or some more gimmicky things. If you need the handheld, you want it to last long on batteries and not draining for fancy pictures of funny ILSes (which are irrelevant in case of anyways as they will clear everything for you). I would go for 833 spacing as there are some parts in the world in need for it and it is increasing. Don't give a thought on certification as we are speaking emergency backup (never heard of enforcement unless you tie the handheld permanently to the plane). Nice to have - keypad and memory, toggle (Icom calls this flip-flop) between last two stations is also helpful and the possibility to charge at a 12V car outlet.
I did change my Icom A24E (never used the VOR function) for an A6E, due to 833 and raise of antenna power from 3.6 to 5 Watts, lately and I feel it is a nice device. I also changed headset to one with Bluetooth and built in power, to have another option in case of.