So I've been using http://planetelderscrolls.gamespy.com/View.php?view=Mods.Detail&id=609 for a long time now, and I usually play without the sneak modifiers because I don't like the armor penalties. This time however I'm playing a character with Unarmored and Light Armor as major and minor skills respectively, so those penalties wouldn't be relevant anymore.

Only problem is, I came to this conclusion after already starting the game without those esps checked. Now, if I activate them my saves simply will not load - the progress bar on 'initializing data' will stop before it gets halfway and that's the end of it. Uncheck them again and everything works fine.

Anybody know of a way to get this working with an existing save, or is there nothing for it but to restart (or keep going without the sneak modifications, I guess)?

Personally, I would not use the TLM sneak modifier plugin as I experienced instability with it before, and I've seen other players report that as well.

Firstly, if you haven't already, try loading the save twice in a row. Sometimes if it doesn't catch on the first try, it will on the next.
Secondly, if you've made previous saves before exiting the game, you might find that your latest save doesn't load but a previous one will. If you get luck with that, try loading your latest save FROM WITHIN the current one (i.e. don't exit the game first). If you get up and running, save right away (under a new name) and try loading that...
Thirdly, try cleaning your save of all extraneous things using Wrye Mash (make a backup first). One at a time, remove debris cells, spawned creatures (if you don't mind resetting all your creatures), and do a Repair All.

I know the Lighting Mod has issues, but I've personally used the sneak modifier quite a bit, even along with the MWE Crime Stealth enhancement, without major headaches. Perhaps mine is just a LLL (Lucky Load List).