I would accept the losses and setbacks if they are realistically portrayed. For example, the Lyrans in the same book. They could hold on to Coventry because they were up against only one overextended Clan.
Then there's your specific highlighting of parts of my posts while dropping the others. I also said they gave us plenty of details on the FWL side of the conflict and their leadership, but it seems the Marians are limited to the III Legio Prefect with nothing from the Caesar.
Being players and seeing more than the characters aside, would experienced military strategists and tacticians really go after Gibraltar over something better? You go on and on about I'm on the outside looking in, but have you actually compared the Marians as portrayed in FM3145 and the Marians in Shattered Fortress? Also, in real world or BT, war factories are preferred targets. The Kuritas made the most of their limited forces by specifically targeting Quentin and Marduk in the pre-3050 period and have been reaping the benefits ever since. The Blakists seized every such world or nuked those they couldn't hold. That's realistic. Going for Gibraltar is not.
Shattetred Fortress gives us plenty of information of information from the FWL's perspective because that's what the in-universe authors had access to. They might not have had the same degree of access to the Marians to figure out their tacical and strategic thinking, their reasoniong and their choice of targets. They might not actually know what was going on inside the Hegemony to the degree that they would like. There's plenty of good reasons why there's a lot of information from one side, and next to none from the others.
I've looked at FM:3145 and see nothing at all in there about the MHAF that precludes the events of
Shattered Fortress. If there's an issue there, you're going to have to be a lot more specific.
Yes, the Marians went after Gibraltar and not Kendall. There could be good reasons for that, but as yet, as players, we are not privy to them. They could still be a part of some greater story that's yet to be fully told. The first thing that comes to mind would be to look at what happened the last time the Marians went after Kendall at the end of the Jihad, which resulted in a complete disaster. They could have delibarately avoided the world for just that reason.
We can speculate on the how's and whys of the situation, but exploding into globs of fanboyish rage and claiming that the authors are ruining the Marian Hegemony for the sake of propping up another factions, with similar accusations of favouritisim is not a useful answer, nor is it supportive of any form of intelligent discourse.
Ultimately, the Marians made mistakes and suffered losses. This happens. This is a part of the Battletech universe, something that has been a part of the fiction since day one. Your faction suffering a loss is not becasue the writers hate it or decided to ruin it for the sake of it.
Now if you want to speculate as to why things happened from an in-universe reason, then that's great. Screaming and crying because bad stuff happened to your favourite faction isn't.