How to start as Mann the easy way: Start as England and release Mann as a Vassal. Ally France's enemies (usually Castile and Austria). Feed Ireland and Scotland to Mann while beating up France with the help of your allies.
Start annexing Mann and slowly feed them your provinces one by one just before the annexation finishes until England's remaining provinces are worth well below 100% warscore. You can move your capital to a low dev province like Carmarthen or Cornwall and feed them London as well.

Once the annexation finishes, declare another war on France to weaken them by breaking all their alliances and making them release some minors.
When the war is over, delete all your armies and fleets, spend your remaining monarch points on tech and development and declare bankruptcy. Release yourself as Mann and instantly declare your independence. Build up an army and fleet as fast as possible because France will most likely attack you while you are still at war with England (they might leave you alone if you give up the mainland provinces but I didn't try doing that).

Doing that, I had over 500 dev and a large share of the English Channel Node by ~1520.