Does anyone have any ideas on how to repair and protect cracked/peeling decals, like these pictured in my 10019 set:
Some kind of film or lacquer?
Oh, those look sad! Stickers with white designs on them are notorious for peeling/cracking.
I have found that the best way to protect stickers like these is to apply a thin layer of clearcoat to them while they are still new. Wait until the clearcoat dries, then apply the sticker to the LEGO set. You can use pretty much any acrylic clearcoat, medium or varnish available at hobby and craft stores. Anything in the scrap-booking/decoupage section should be safe, but it is a good idea to test them on a small corner of the sticker-sheet. Always apply the clearcoat thin. You only need a thin layer over the sticker. If you apply too much the clearcoat can yellow with age.
Once stickers are already damaged it would be pretty much impossible to repair them. Even if you try to put a clearcoat on an already damaged sticker, it will continue peeling, so it is not worth it. It is much easier to just get and extra sticker-sheet for any LEGO set on Bricklink. The stickers for your set above are here.
I have actually made it a habit to always pick up 1-3 extra sticker-sheets on Bricklink when I buy a new set. While the set is still new and readily available the sticker-sheets are much cheaper (often just a few pennies) than trying to get a pristine stickers-sheet for a set that is 10 years old. Just place them in a ziplock bag, and store them away from heat and direct and sunlight. They won't deteriorate that way.
Replacement stickers from a third-party site are an option, but they can be expensive - the sticker sheet for this set is currently between $75 and $200!
You can repair printed paper stickers, if you want to take the time and care to do it. The basic steps are: