Scenario 1
It sounds like you are assuming a "pure" pursuit on the enemy ... he then pulls a split-S and you still follow with a "pure" pursuit split-S.
What I would do under those circumstances is nose down enough to let him know that I am still coming for him, but keep the pursuit to more of a "lead" pursuit for the "just in case".
Once he does the split-S, I can choose to follow, or I can pull off and up to maintain alt dominance.
If I decide to follow the split-S, because I was in "lead" pursuit, my split-S does not have to be as drastic as his was and if I did it right, I should have no problem pulling off the split-S and maintaining my advantage behind him.
Scenario 2
Again it sounds like you are assuming a "pure" pursuit on the enemy.
Because you are in a Spit, and it sounds like you don't really have enough smash to overtake him, and you won't overtake him, you should be in a passive "lead" pursuit.
Once he goes nose down, I would follow with a slight nose down (still in "lead" pursuit) to keep my E up.
Now, because I have maintained a "lead" pursuit, once I see him pull for the zoom, so do I, and because I was in "lead", I can theoretically "cut him off at the pass". I may not be right on his tail in the zoom, and he may be pulling away ever so slightly, but I have yet to see any plane within D400-D600 out zoom my 20mm or .50 cals.
The object here was for me to maintain a position that would give me the chance to make things equal and give me the chance to get a decent guns solution to get a shot off.
A lot of Air-to-Air combat, in this game, has to do with anticipation and positioning yourself correctly for what you anticipate. What you anticipate does not have to be a single anticipated action ... decisions and positions could be based on ... he may do this or he may do that ... so I will do this to cover both of the outcomes.
The more you fly ... the more fights you get in ... the better your anticipation skills become and so will your decisions.