How do I know when I'm prepared to begin working on Manna progressions?
Forget about manna till you can do a straddle-L. On that note, you need to be able to do a decent L-sit before you even think about manna. Shoot for 60s.
Similarly, with Inverted Pullups, how do I increase the difficulty?
Simply, you pull higher. Straight body is a tad more difficult than tucked.
70'sBig has a basic strength and Oly WL program. You could train that if you wanted to train the Oly lifts. Makes sense if you want to have a vertical. There is a lot to be said about just doing SS till you peak out.
Note that Olympic WL is hip dominant and that is detrimental combined with the fact that you are tall.
As for the levers, do them if they make you happy but having a solid body lever and L-sit and HS are more beneficial in the beginning.
Press HS is merely about working the HS and then the Headstand presses into the HS negative presses.