Canon wars!!:devil:
Just out of curiosity, is the Errata 7 considered canon or not? 'Cause, I'm thinking if this is an argument over what IS, and Errata's canon, then there's nothing to argue over: Regina's TL12 per Errata, end of story. Which is not how I would have handled it - explaining to a player why the data in his deeply cherished, much beloved, carefully protected vintage copy of the rulebook isn't really the rule anymore, is just a pain in the kabongas - but I don't get a vote on stuff like that; I'm just some guy with way too much free time on his hands.
However, if this is an argument over what SHOULD BE, then it SHOULD BE whatever suits your fancy. Some of us like TL12 1105 Regina but, clearly, some of us don't.
I don't recall a rule that allows you to split forces in the way you describe - I was under the impression they stacked and you use the tables for the combined factor...
FFW: Regina has a 1C-10 Colonial corps marker, little red square with white print, available for transport to other worlds. In addition, Regina has 15C (1500) native defense batallions reflected on the game map. Two distinct units. Meanwhile, the 4518th is set as six separate 1-batallion units - tough little puppies, 5 of them are armored and all six are elites, so the six of them are worth 22 regular batallions.
"A firing unit may split its combat factor to fire at several units. A unit is not required to fire during surface combat, but enemy units may attack it even if it does not fire. Each unit is attacked separately; two or more units may not be attacked in a combined attack. ... There is no requirement that all enemy units at a world be attacked ..."
The net effect is that the Regina force has quite a bit of latitude as to how to divide its attack. Certainly the 1C-10 unit can divide its attack among the 6 (or fewer) Marine units while the planetary batallions sat the battle out - but that would not save the planetary batallions from being attacked in turn, so there's not much point in this context. Makes more sense if there's a TL difference among attackers, since the combined attack is governed by the tech level of the contributor with the lowest tech level; you want that low tech unit sitting out when he reduces the odds too much for your high tech unit.
So ... sure you can, but there's no point in it.
Basically you've got 1600 batallions against 6 elites, 5 of which are armored. At TL-10 - and unless they are unusually lucky or reinforcements arrive (barring bad luck, the battle takes more than a month to conclude, adequate time to send for help if you've got a ship with good "legs") - it's Thermopylae, not Rourke's Drift. Marines lose, but roughly half the Reginans on average go home in pine boxes after a fight that's going to become the stuff of legends. Of course, that's not factoring in the SDBs or the ten Colonial cruisers at the naval base.
Politically, I can't help thinking the Reginans'd be having second thoughts after the first couple hundred thousand casualties; losing over a thousand men for every enemy APC destroyed tends to sap morale. However, I still haven't quite figured out what would persuade ALL the planet's native forces AND the colonial expeditionary corps to turn against the Duke, especially when the likely result - win or lose - is an Imperial fleet overhead while additional Marine units land to squash the revolt and track down the ringleaders. Does not do to let people get away with that kind of thing, even if they have the best of reasons - at least not in my TU. Strikes me that with the full weight of Imperial authority behind him, the Duke doesn't need to restrain the local tech advancement - and without it, the Duke should be more concerned with external enemies than internal dissent.