First of all, you should not rename MainLSF.pak into anything that has the extension .pak but e.g. into MainLSF.pak.original.
(The game engine picks up root templates regardless of the name of the files they are stored in, as long as they end in .lsb, so there is a good chance it does this with PAK files as well.)

Additions to ItemCombos.txt are picked up by the game no matter if it is a new game or if a save game is loaded, the recipes are read on every game start, so new ones are picked up.
(At least if the recipes use stuff known to the game, which is the case with the mentioned recipes.)

If the ItemCombos.txt is the only file in the Mod (it's not a Mod technically, it's from the Pre-Editor days ;-), I see no problem and no reason why it should not work. (Except if a preview file is required, and I don't see one in the extension. I had this problem last year. General Raze would have to answer that.)


Little bit Offtopic:
I'll take the chance of the presence of the General himself in this thread to ask some technical questions (because I don't expect any 'Larianian' to ever show up in the modding forum, not even HIM ;-)

In Original Sin EE (as in probably every crafting mod) I always find lines like:

use object LOOT_Ink_Pot_A_Quill_A_Magic on object Scroll_Skill_Fire_Blank_A with transform code Transform2 parameters SCROLL_ImmuneToFreeze,0,0,0,0,0,1,1 requires Crafting 2 autolevel 0 autoValue 0
use object LOOT_Ink_Pot_A_Quill_A_Magic on object Scroll_Skill_Water_Blank_A with transform code Transform2 parameters SCROLL_ImmuneToBurning,0,0,0,0,0,1,1 requires Crafting 3 autolevel 0 autoValue 0

which means that writing on a fire scroll may result in an Avatar of Frost scroll and writing on an empty water scroll may result in an Avatar of Fire scroll.
In Classic, that was okay, because the old Target_ImmuneToFreeze skill was a fire skill and vice versa. Not anymore, in EE they are in the 'correct' schools but crafting was forgotten in the change.

So my questions are:

1. If I put my own ItemCombos.txt into Data\Stats, are all other crafting recipes still read ?
(Since crafting is very important for story as well - the Death Knight Bane skillbook as an example - and most mods only add the Shared part and forget to add the Main part of crafting, I suppose the answer is yes.)

2. Is there any way to disable an existing recipe, for example by simply changing the use code of the lines above from 'Transform2' to 'DoNothing' or would that just add another possible output ?

3. If recipes are overwritable in this manner, do I have to keep the original order of recipes in ItemCombos.txt ? (would it have to be on the same line number x ? Don't laugh, everything is possible, the story editor's search function works in such a weird way.)

4. Do recipes work without a preview in ItemComboPreviews.lsx ? Always ? (my experience last year was different, but we are some patches ahead ...)

5. Can I use both internal AND external ability names in the requirements ?
- 'requires Blacksmithing 2' uses the 'game name' (engine uses 'Repair')
- 'requires Blackrock 3' uses the 'engine name' (game uses 'Tenebrium')