I'm fairly new to printing as well, but one thing I have already found is that once you get the "feel" for your rig, you will find that it all gets very consistent. As you say that a developed paper with no exposure was OK (except for a tiny bit of fogging caused by the very-easy-to-do mistake of turning the light on with the paper box open ) the only way you are getting black prints is over exposure. I would set-up for grade 2.5 and run some test strips off a neg from say 2 seconds right up to 20 seconds. That way you are going to see straight away where to start.

If you are using ilford chems, I find their universal developer does keep at working strength for at least a week, and seems pretty OK even after 2, but definitely a week is OK in a closed bottle. Possibly the super-critical might see a difference, but for me, while I'm learning, week-old developer works fine, then I junk it and start again.

BTW, I wouldn't bother with concertina bottles. Nobody on here seems to have a good word for them - I bought a load of 500ml HDPE bottles off the 'bay, and keep all my working solutions in these. A full bottle is just right for my 8 x 10 trays.

re the focus scope. Something sounds wrong - it should be easy enough to find the grain - focus without the scope first, line the scope up and fine tune. It should be that easy.