Food safety of sauerkraut whose brine evaporated below surface?
Unfortunately nobody can give you a definitive answer about your batch without testing. But I can give you more information to help you make a more informed decision.

As you are, I'm concerned about botulism. You mention that "botulism can't survive with that amount of salt". I'm not sure whether that's true. If an environment is salty enough, or if it's less salty but acidic enough, you're all set. But the question is what's enough; at some point it's far less likely, but not impossible, for clostridium botulinum to survive.

Your primary defenses were (1) a salty, acidic environment below the surface, and (2) oxygen at the surface. It's hard to know whether the cabbage tissue at the surface was anaerobic enough; maybe so. As for acidity at and near the surface, if mold was involved somehow that could lead to a drop in acidity. From Sandor Katz's excellent The Art of Fermentation, p. 50:


  Molds can also digest lactic acid, lowering the acidity of the ferment that enables it to be effectively preserved.


So...


if the surface environment became tolerable for c. botulinum, and 
if it grew in that environment, and
if it produced botulinum toxin, which then got into the submerged kraut


then as I understand it, boiling and canning won't change the presence of that toxin unless you boil it for at least 10 minutes. (Thanks to Josh Caswell for pointing this out in the comments).

On the other hand... that is a whole lot of ifs. 

I'm just an informed layperson, so you'll need to make your own food safety call based on this and information from others. My guess: your kraut is just fine. 

But because I always err on the side of caution, in the same situation I'd probably chuck the batch (with tears in my eyes). Though not in this exact situation, I've done it many times, and it hurts, but I sleep better, especially since I share my ferments with others. 

Or if your "hot pack" will heat the kraut long enough to address both botulinum spores and toxin&mdash;I know nothing about canning&mdash;you're probably fine too. I'm not sure what that does to the kraut's texture, and you lose probiotic benefits, but it sounds like addresses the safety concern, at least for botulism.

Some more sources:


The CDC has lots of info on botulism, as does Seasoned Advice.
The Three Biggest Fermenting Mistakes You're Already Making has a concise list of fermentation warning signs.
NC State University has a safety sheet on fermenting vegetables.