I had a short chat with RizZen in Discord about the new clan invasion campaign. So. Answering here to my questions myself.
1. The new campaign will not be connected in any way with the previous developments.
2. There will be no more updates to the 4.x branch. Version 4.2 is final. In particular, there are no plans to tweak some of the fragility of the balance in later missions, which are mostly done by combining a large number of Mad Cat's. If you don't like that fact - you can download shorter separate campaigns here: https://www.moddb.com/games/modding-mechcommander/news/mc-beginners-guide-all-mechcommander-versions-overview
All from 5 to 30 missions in total.
3. About 200 unique missions will be released.
4. LipSheZ missions (Repulse project) will be incorporated into Prometheus.
For updates, in particular, you can follow the large Discord channel Everything Battletech - https://discord.gg/ZcjCW7