Ooh, that's massive, actually! If you could enable logging (it should be easy to find a tutorial online), that would also be HUGE. As for how to speed things up, around like 319 of Kipchak_Functions.lua, there should be a line like this:

table.insert(KipchakMercenaries, {iPlayer, pUnitType, pUnit:GetExperience(), (Game.GetGameTurn() + 25), selectedPlayer, pNewUnit:GetID()})

Change "(Game.GetGameTurn() + 25)" to something like "(Game.GetGameTurn() + 3)" - that +25 is what controls how long a unit takes to return.

I increment my save files, so i have like 30 I could comb through manually to find around where it starts happening.
Is there a setting in the mod files i could alter to make the unit return interval less to like 1-2 turns (instead of 30) to help speed it up?

I appreciate the help, but it's unfortunately really difficult to tell what the issue could be without logs, even more so when there are other mods active. If it happens to more people or I have time to really dig into it I'll try to sort it out, but as things stand right now I don't even really have anything to go off of, so I'm not sure I can fix it. So sorry for the trouble, I'm really not sure what the heck could cause something like this!

I didn't really notice when unfortunately, I had been spamming warriors and archers early on, sending half away to mercenary and keeping half at home. And only really noticed at some point when i sent a spearman away, and noticed after a big war/upto 100 turns later, that it didn't come back.

Less Ice on Maps (v. 101)
Less Warmonger Hate (v. 10)
Lighthouse Near Sea +Lakes (v. 3)
Lungora and Nope Copter's K... (v. 1)
Maps Change Start Location (v. 2)
More Luxuries (v. 155)
No InstaHeal (v. 1)
No Pantheon Cost Increase (v. 1)
Promotions Expansion Pack (v. 9)
Quick Turns (v. 10)
R.E.D. DLL for BNW (includ... (v. 9)
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Religion - Improved Custom... (v. 15)
Religion - No Al Free Pottery (v. 1)
Resource Granary Include... (v. 3)
Ressource Mint Include C... (v. 2)
Speedy Starting Settler (v. 1)
Terrain Poor Tiles Tweak (v. 2)
Tweaks- Supply production ... (v. 1)
UI - City Happiness (v. 17)
UI - Map Pins (v. 17)
UI - Overlay City Limits (v. 8)
Various Refinements
Workable Mountains (v. 2)

Yes, lots of mods

Active Mods
(3UC) - 3rd Unique Component (v. 7)
(4UC) 4th Unique Component (v. 10)
Artificial Unintelligence Lite (v. 9)
Barbarian Spawn Increase (v. 2)
Barbarians Unlimited Exp (v. 2)
Captured Settlers Remain S... (v. 1)
Citadels don't steal land (v. 3)
Cities 4 Hexes Away (v. 5)
City Limits (v. 7)
City-State Enhancements
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Dynamic Towns (v. 16)
Eras Classical Age (v. 33)
Ethnic Diversity - Steam v... (v. 29)
Improved Lighthouse (v. 2)
InGame Editor+ (v. 46)
Jake Walrus Whale's Venice (v. 4)

Sorry, I genuinely don't have a clue what the issue could be, so it's hard to even start, especially since I've never had anybody else report the issue - I'll try to find time to look sometime soon, but I'm honestly just confused. Were you using any other mods? Do you have an idea of approximately what turn or even what era it occurred at? Any other potential information?

is fixed yet?

Huh... that's strange. I'll try to look into it, not sure why that would happen. Thanks for letting me know!

The mercenary ability stops working midway through a game (not sure when, i didn't notice), but was thinking some of my units didn't return since a war or to, and now realised that my garrisons cant go mercenary at all.