Ok - a quick stab at an answer from a snowy Newfi
Could it be that loss of one of the primary nav systems (an IRS) renders the whole system 'defunct' in terms of failure logic and that for 'safety' your system simply says ' you are on your own, matey'? As Captain S says, to answer another of your queries, it is
normally the left IRS in the Classic which is the key (in your case
not available), and I wonder whether line selection of the right as 'default' in your incident might have restored the pos?
Your tech pilot should be able to establish the system logic for your fit, I would have thought?
I'm not sure Bill B will help in this case as you appear to have a 'one-off' installation as far as I can see, although it is an excellent series to own for everything 737.