Hello Darian,
I am biased when it comes to linen backing and restoring a poster that is damaged with tape residue, staining, fold separation and missing paper. A piece in that condition can look so nice when restored properly, but this takes the talent and skill of professionally trained artists.
You can certainly improve the appearance of your posters by filling in the lines with some pencil crayon or water color, but to match the color perfectly takes skill and experience.
As for bleaching, we do this as part of the linen backing process, since the mild bleach solution is mixed with water the poster ultimately will get wet. Once wet, the poster will have to be dried properly or mounted on linen, not something I would want to be doing with a valuable poster if I had never done it before. At very least, I would stay away from experimenting with bleach or any other cleaning solution as the results could be disastrous.
Please let me know if you have any additional questions. I want to ensure your collection does not meet with an unintentional accident. Email me anytime at backingtothefuture@gmail.com or call me at 905-213-1476.
All the best, Dan Miles, Backingtothefuture.