Well, the first thing to check the next time you go out to it is the fuel pressure! Theyre pressure operated injectors, and if its low its going to start and run like a bag of nails. Before you even turn the key try the flap, it wont feel like a massive resistance, the tell tale sign is how it rises again, if it springs back and bounces then there is no fuel pressure. You could also crack a nut off after, see if the fuel sprays out.

My cabby started OK ish from cold, but stop in a petrol station once it was hot 3 minutes of cranking later......

If it is low when the pump primes you'll notice it change tone as the pressure builds up.

Your oil cap - normal, but is the exact reason mine is disconnected, too much to make air leaks on an engine.

Ah right, i know nothing about gauges to be honest! It should be safe to run it off the housing, but like evrything take it easy, dont just ramp it and take it for granted. I expect its your gauge to be honest. but i'd get the thing to start first before trying that, that wont affect how it starts.