Bleach Bath Timing?

I've posted before about bleaching my hair and then using One n' Only Colorfix because I'd forgotten and my ends were still a bit red from a box auburn dye job.  The result is an uneven mix of pale yellow, dark gold, and brassy sections that disappear for a day if I use a combo of Shimmering Lights and Joico Color Balance Blue, as seen here:

^ One day after toning shampoo

^ The rest of the time...maybe because I put conditioners and masks and oils on my hair all the time.

In addition, my undercut pixie probably isn't the best style for going radically lighter, as I imagine it'll be a challenge to maintain once I get it to the near-platinum shade I'm aiming for:

^ As you may be able to see in this poorly-lit pic, the undercut is actually a lighter, and more yellow, shade than the rest.

On to my questions. In about a week, I plan to try a bleach bath, since my hair isn't feeling stiff or gummy when wet or fried when dry.  I really want to get a couple of shades lighter in the gentlest way possible (and I'm also hoping that using the purple and/or blue shampoos will be enough for toning, unless there is another suggestion? I have a box of Wella t14 but would rather not use it for a few months at least). 

My concern is timing the bleach bath given the uneveness of my hair color at present.  I know to do the ends first, normally, but I'll also have 1/4 in. of dark brown regrowth on the top of my head (which won't lighten enough with a bleach bath) and some hair that is much lighter than the rest.  I don't know how successful I'd be a spot bleaching, so would it make sense just to apply the bleach bath as quickly and evenly as possible all over, instead of staggering it from ends to roots (except the undercut, which I'm guessing should definitely go last)?  In addition, my hair is a bit too short and thin to do a decent strand test (cutting anything off would be noticeable) so does anyone have an idea about how many minutes the bleach bath should take to lift a shade or two (using Wella lightening bleach and 20 vol and some sort of clarifying shampoo)?