On the different game, same version number thing: That’s a tradition that dates to the mid-90’s with the 3 games published under the title Mechwarrior 2.

31st Century Combat was the first, it featured two campaigns from both sides of the Wolf/Jade Falcon Refusal War. Ghost Bear’s Legacy is also post-Clan invasion but largely to do with the Draconis Combine. MW2 Mercenaries is set pre-invasion up through the Battle of Luthien.

Mechwarrior 4 was fairly similar; Vengeance was a relatively small story set on Kentares IV (and its moon) and is kind of a microcosm of the FedCom Civil War. Black Knight does continue the bad ending of Vengeance, and MW4: Mercenaries is more broadly about the FedCom Civil War; most missions are either Davion or Steiner aligned though other units and factions appear (including the Jade Falcons and the Capellans). Kentares IV isn’t so much as mentioned.

So what does the Inner Sphere bring you for serious fights then?

This is something I don’t think any of the Mechwarrior games ever really brought to life because yes the Clans had outright superior weapons and the Inner Sphere to my knowledge never won a toe-to-toe fight during the invasion. Name one time an inner sphere lance stood against a Clan star in a fair fight and won. The clans ultimately lost because it turns out blitzkrieg is a dumber thing to base a religion around than the phone company. And it’s really difficult to build an action cockpit simulator game around that as a primary gameplay mechanic.