The link is posted above in my comment

Started reserving seats with our Global Passes through Austrian Rail (OBB). Apart from annoying problems with logging in — they keep not recognising my email address (or password) even though they use it to send me tickets — it’s worked okay up until today when I tried to reserve seats from Salerno to Sicily for 5 April. Initially I tried Messina but that wouldn’t work so then tried Siracusa and Palermo, but no luck there either. Any suggestions please?

Probably a bit too early to book. Italy is a bit late in uploading and sharing its timetables with the rest of Europe.

On Italyrail and Trenitalia I find connections for that day. Precis are low, so I don't suspect trains to be fully booked. So no worries, best check a couple of days later if they show up then and reserve then. 

Thanks. We didn’t have any trouble booking as far as Salerno but it must be the north/south divide that our Italian friends talk about. I was worried that maybe seats were sold out because of Easter.

I assume you are looking at the day train, not the sleeper. Last year I did the reverse journey and it was indeed only a few weeks before departure they appeared on IR/OEBB for booking, so as above post, just keep checking - the direct train is an IC not a Freccia and crosses the Messina straits on the ferry.

We managed to reserve seats today for travelling by trains in Sicily but not sure if we’ll be sitting together. We have 1st Class Global passes. From Messina to Siracusa on 6 April at 16.05 we were given seats 13C and 12C in coach 1. On the 10.00 train from Palermo to Messina on 20 April we have 2A and 2C in coach 8, and on the 10.10 train on 21 April from Messina to Sapri we have 2A and 2C in coach 1. 

I suspect that we’ll be okay with the Messina to Siracusa seats but can anyone tell us if the other seats make any sense? We really don’t want to be sitting on the other side of the aisle from each other.

Hopefully the man in seat 61 has the answer for you:

https://www.seat61.com/european-train-seating-plans.htm

Thanks, but I’ve looked at seat 61 but the InterCity seat plan looks a bit old and doesn’t seem to cover what I’m looking for.

Trenitalia site suggest seats C and D are a double together, but A and C are on opposite side of the Aisle. On the few times we have been on these trains there has been plenty of spare seats to move.

Thanks. Didn’t think of checking out Trenitalia. I guess there’s no way of changing seats through them? I reserved them through OEBB.

Since they are only 3 euro each you could always check at the stations in Italy about a change and, if they can’t change them simply buy a couple more.

Also if you have the required reservation reference you could try this page:

https://www.trenitalia.com/en/purchase/manage_your_ticket.html