When I worked for Ilford (1974 to 1976), Ilfosol was known internally as "Ilfoslop". It used to be supplied in bottles which were full to the absolute brim, the reason being that it was acutely susceptible to oxidization. I tried Ilfosol at the time (I think Ilfosol S is a later formulation) and found it good if you cracked open a fresh bottle, but any developer left in a part-used bottle "died" within a day or two. I have not tried it since, so cannot say if it is still as susceptible to oxidization, but it could well be.
Another test for your own piece of mind might be to expose your lens/shutter to cold for the same length of time as you did the day the shots didn't come out. This could be achieved by leaving your camera bag in the car trunk. It is conceivable that the shutter is making, for example, a "1 second" noise in terms of whirring gears but without actually opening!
Exposure through the back of film generally produces a faint image rather than nothing at all.