The top and back seemed to go on thinly and evenly, but on the sides it just didn't seem to want to stick. It was evaporating off as quickly as I was wiping it on.
I can understand a different appearance on the end grain, but not the sides where the grain is running the same as top/bottom.

Also I would say the poly mixture is absorbing not evaporating (or at least should be). Presuming you use the right solvent, the UK equivalent to turpentine not something like denatured alcohol (aka methylated spirits) which will genuinely evaporate rapidly. However I'm not sure that poly and alcohol would mix without a very obvious and immediate reaction.

With the end grain, it should eventually start to build up and look the same as the long grain. It would certainly have a different appearance with only one coat. Wipe on poly (and Tru Oil) will build up slowly as each coat are literally only microns thick.

Also, another noob question. Of course, I mixed way too much – how long will it be usable for? I wrapped it in an airtight plastic bag to try to prolong its life.
IME, my batches have lasted anywhere from a few days to a week+, but our respective climates a are quite a bit different. I have no idea why mine have gone off at varied rates, and the end results range from an obvious change in colour and slight thickening, to a complete gelling.