New colour plan, got a few questions

I would do them in separate sessions - bleach first, wash it out, then pin up the longer hair (with a conditioning hair mask perhaps) and apply dye to the undercut with a tint brush to avoid overlapping.

1. Would I need to do a full strength bleach, or could I achieve my goal with just a bleach bath?
I'd do a full bleach.

2. Regardless, which strength peroxide would I need?
A good quality bleach, not a cheapy one, and 20 vol. maybe even 10. Remember to do the roots last.

3. Will I be able to achieve my desired result with just a single session of bleach?
Almost certainly.

I'm also planning on shaving in an undercut, which I want to dye permanent black, and keep fresh and vibrant with black semis. What I was thinking of doing was applying the bleach to my lengths and then to my roots, and applying the permanent black to the shaved part as soon as I've finished applying the bleach.

4. Would this be a good idea, or should the bleach and the permanent dye be done in two separate sessions?
Separately.

5. If the answer is the latter, can both be done on, say, the same day? Or would they need to be spaced a week/month apart to avoid accidental double processing where the longer hair and shaved hair meet, if you understand what I mean? *stars*
Same day, but make sure to wash the bleach out avoiding getting the bottom hair wet, hair over the bath or something. The dye the bottom and wash out in the shower, stood up, avoiding the top hair.

Thanks both 🙂

I have another question; if I end up with an uneven base after bleaching, would mixes of reds/purples etc. cover it evenly enough?

I have found dark pinks and pink based purples to be quite forgiving over uneven bases. Since your hair is virgin I can't imagine you will have a problem 🙂

Yeah, I agree!