I wouldn't mind if it was portrayed as what it really was, greedy nobles vying for an irrelevant throne. But that somehow Arano is a hero for further destabilizing her nation and bringing her entire sector to the brink of war for some nonexistent reasons other than "I'm the legitimate queen!" is ridiculous. If anything, if she cared about her people she should have stepped aside. Instead she decided to obliterate the armed forces of her own country (good luck mustering the same troops you needed against foreign powers now), razed her own planets, and killed off her own leadership.
Yeah I think you hit the nail on the head there as to why the campaign feels so hokey. Admittedly, I've never read, seen, or played anything Battletech besides this, MegaMek, and Mechwarrior 4: Mercs but I always got the impression that 99% of the wars that happened in the setting were pointless.
Also, yeah not being able to betray was also terrible. I spent the whole game telling Arano that I just wanted money and I REALLY did not care about her but I was stuck in anyway. I really enjoyed that mission in Mechwarrior 4: Mercs where you could blow up the truce your employer was setting up after a series of missions just so you could get a ton of money from the Capellans instead. The whole campaign in Battletech made me feel less like a mercenary commander and more like a retainer to Arano mainly because I didn't have choices like betraying Arano. I might not of betrayed her anyway, but having the choice would've added a lot for me.
This is the Inner Sphere in a nutshell. A bunch of neofeudal assholes squabbling over the ashes of a throne that ceased to exist centuries ago, interrupted by a brief interlude courtesy of the resident barbarians, who were quickly Romanized, thus allowing the expanded party to get back to the same old petty bickering with slightly improved technology. It's not as static as 40K, but it's close.
As for the plot of this game, you're an Arano retainer not only functionally but literally. Remember, in the tutorial
you're vetting your 'mech with your co-worker, the head of her personal guard, preparing to escort her to a high-profile political event. When shit goes down you are one of the two people she implicitly trusts that isn't a family member. If you want to read a little more into it, the rescue of you and your 'mech wasn't coincidence. It (potentially) didn't just so happen that a random band of mercs who just so happened to be wanting to recruit a leader also just so happened to be in orbit when the coup went down and just so happened to rescue you.
If you want to be cynical about it, Arano used a third party (possibly a plant in the merc crew) to arrange your rescue, as you'd proven to be one of a handful of people she
could trust. Odds are she paid off at least one of the officers of the merc company to guide things toward placing you in charge, giving her an independent and deniable force firmly in the hands of a member of her personal guard.
Practically speaking, very little time is spent as a pure mercenary before she re-establishes contact. The story is functionally of [player], the retainer of Arano, posing as a mercenary to conduct grey- and black-ops missions against her uncle, rather than [player], the mercenary, hired to help her retake the throne. Frankly the "gimmie lodes of monee" options are only there to provide the illusion of it being an actual merc campaign.