As far as I can tell there is no way for the game to 'lock' a recipe.
So they don't need 'discovering', you can use them right away with enough crafting/blacksmithing or whatever they require. (If they technically work of course, which is obviously not the case for you.)
I have only looked into the file a little bit not completely, maybe there is one or more error(s) in the file that makes the game ignore it completely. (In some cases, because it seems to work for others if I understood correctly.)
As far as I remember I unpacked the complete Main.pak, put the unpacked files into the correct folders and removed both MainLSF.pak AND Main.pak. (Would not recommend to do that now, the times when some 'drastic measures' are needed are over since the release of the editor.)
My ItemCombos.txt file back then was
F:\Steam\...<DOS-EE-Directory>\Data\Public\Shared\Stats\ItemCombos.txt
