I'll address it more fully once we get to those scenes, but I agree that the whole setup makes no sense at all.
[Scene 1 - on Stein's Folly] [Pavel Ridzik in closeup, as he triumphantly steps across the corpses of Davion soldiers. In the background, two majestic
Overlords disgorge heavy 'Mechs.]
[Scene 2 - on Sian]
[Ridzik]"Greetings, oh Chancellor, First Lord, Divine Wisdom, Sun of my Heavens, blah blah yadda yadda. I have burned a command circuit across seven jumps only to
gleefully continue my adulterous relationship with your wife personally report to you on our progress at Stein's Folly. I'm happy to report there's nothing to report. Really. And if there was I wouldn't tell you."
[Liao] [twirls moustache] "There's this one close friend Davion has, one Ardan Sortek... I'm just mentioning this."
[ComStar messenger] "Bad news from Stein's Folly."
[Liao] [hissy fit] "RIDZIK! Get to Stein's Folly immediately and in person, burning my command circuit, and shoot anyone or anthing opposing my grandiose plans!! Then get back here and tell me how we fared. 'Cause that's obviously cheaper and quicker than ComStar."
[Scene 3 - on Sian]
[Ridzik] "Bad news from Stein's Folly... again."
[Liao] [hissy fit] "RIDZIK! We've been betrayed by the Davion traitors! How could that happen?! Now that the trap didn't work we don't have enough troops on Stein's Folly to hold the world and must withdraw. Btw, Ardan Sortek was with the attackers."
[Ridzik] "Indeed. Now that you mention it, we've downed his 'Mech and noticed it fought just like Sortek, but somehow didn't consider this important until now.
[Ridzik travels to Stein's Folly again, apparently using the command circuit again. This isn't worth a new chapter, indeed the very next paragraph begins with Ridzik having arrived on Stein's Folly already and ordering his people to secure Sortek.]
[Sortek] [Ends up tied to a tree and captured by Liao troops]
[Ridzik] [makes odd remarks about how important Sortek is, when really, he isn't]
[unknown spy reports Sortek's whereabouts to Davion commanders, who rescue Sortek from an almost abandoned field hospital where the Capellans somehow left him behind]
Now here's my conclusions from this:
This whole episode is quite obviously a setup, a Disneyland Ride designed to let Sortek see a double of Hanse Davion before being rescued by Davion troops. Nothing else makes sense. And it is a shabby setup, probably because Ridzik and his men had little time to set this up once Sortek was found.
Why was this done? Because it makes sense, in a devious reverse-psychology sense.
The entire Doppelganger ploy is somewhat contrived. But now imagine this: The disenchanted childhood friend of Hanse Davion, after a mild falling-out, goes into battle and returns with a case of severe PTSD, hallucinating about a doppelganger that must have replaced Hanse Davion. In this way, the Doppelganger idea is easily discredited and other people will look the other way hard instead of siding with Mad Sortek. It makes it all the easier to believe that Hanse Davion is of course not a doppelganger even if he behaves strangely.
I didn't see any specific reference to the St. Ives Armored Cavalry, though since Ridzik was there in person, it wouldn't be a much of a stretch for Stapleton's Iron Hand (from the St. Ives Armored Cavalry) to be there, as his personal unit. The Death Commandos were noted as being there alongside the Big Mac.
I think the Liao sourcebook says that by 3025, the Big Mac was back to patrolling up and down the Liao/Davion border, spoiling for a fight.
In Ridzik's first scene on p. 28, it is described how the Fire Lance of an (otherwise unspecified) company of House Liao's St. Ives Armored Cavalry - two
Ostrocs, a
Catapult and a
BattleMaster - disembark from one of the two
Overlords at the Steindown port. In the next paragraph, a MAC lance (another
BattleMaster, an
Archer and two
Trebuchets) is mentioned. Quite an impressive lineup in these two lances. Go Liao!
The line about the MAC you're quoting doesn't rule out them being on garrison and strategic defense duty in 3025. And in this role they were usually broken down into company-sized units. Hence my assumption that no more than one company (or three, filling one
Overlord) were diverted to this offensive. Certainly not all five regiments, if the MAC had even regained that strength by 3025.
The Death Commandos destroyed the early warning system at the space station. There's no mentioning of them deploying BattleMechs on the ground though.
If there are two (maybe three) regiments on Stein's Folly, after some were withdrawn, then the initial invasion must have been at least three or four regiments. Presumably the ones pulled off to deal with other problems didn't go back to Redfield, implying there were more than five regiments involved in the joint Redfield/Stein's Folly offensive. I can see the Big Mac and Death Commandos being used in the initial assault, backed by Confederation Reserve Cavalry units, and then having the Big Mac and Death Commandos pulled away to put out fires elsewhere, leaving just the CRC mooks to hold the planets. Since the AFFS crushed the Stein's Folly garrison, whatever unit it was is probably lost to history, and wasn't listed on the House Liao sourcebook TO&E for the CCAF in 3025.
(There's always my pet theory that the Shin Legion was knocking around during the 3rd Succession War, but just got left out of all the reporting. If it was splattered on Stein's Folly in early 3025, and subsequently reconstituted, that would explain its absence from the roster. There's that cryptic reference in the Warrior Trilogy to a Solaris MechJock being a "member of the Lost Legion," that was defeated by House Marik on Shuen Wan. Since the Shin Legion was the only unit in the CCAF with a "Legion" title, and there's never been a good explanation for when/how the unit was formed, I like to imagine that the "Lost Legion" was a disgraced regiment of the Shin Legion.)
Hm. A later scene seems to suggest the MAC is in fact the core element of the trap that is to be sprung on Stein's Folly so perhaps at least a battalion after all. But of course Davion wouldn't know it's trap, and would not take those units into consideration.
However, in the same chapter it's also clearly said that the Liao forces are outnumbered by the Davions after the initial trap failed, and cannot hold the planet. So in any case, there were less Liao than Davion troops there despite the whole situation being a Liao trap.