Oh boy, some breaking up of the paragraph would help a lot
It's very hard to read a lot of text like that, especially with questions in.
"Should I wait til the semi-permanent color in my hair fades before I try to lighten it more evenly again or can I do it now? Do I have to remove the purple and blue before bleaching it if I'm going to use the same purple and blue after I lighten it again more evenly?" No, you don't HAVE to remove it, but it's better to get rid of as much of the colour as you can. There's a few reasons for this, but these are the main ones.
1, purple can have an adverse reaction to bleach. It can turn green.
2, you need to be able to see as much of the depth of colour that your hair has without the colour on top so you can accurately spot bleach the darker areas.
"Does anyone have tips on how I can lighten my whole head more evenly if my hair is darker in some spots than others?" You need to spot bleach those areas. Basically, pick out the darker spots and put bleach on it. You'll want a low volume peroxide as you don't want it to work too fast or be too strong as it's already partially bleached.
I do have a question though, was your hair dyed already? Hair getting lighter on the ends than the midshafts and roots is unusual.
"The last time I bleached my hair the process of lightening seemed to slow down cause the bleach on my hair felt like it was drying out, is that possible and how can I prevent that?" Making sure you're generous enough with the bleach, don't put it on too thin. Get more bleach if it's going to be thin. Making sure you have the right ratio of powder to cream/liquid peroxide, there should be instructions on the pack. Making sure you have a good bleach is another. Don't work close to a heater. If it looks like it's going to dry out once it's applied, wrap it in cling film.
