What did Krillen vs Vegeta show?No. We're shown directly with Krillin vs Vegeta that it doesn't work that way. The Freeza and Cell fights as a whole don't work if it works that way.
Spirit Bomb is explained by it not being strong enough.
How did Vegeta survive Great Ape Gohan falling on him if durability is only based on current power level?Krillin vs Vegeta showed that durability correlates with your current power.
Stamina loss is a different thing. Cell can beat Goku even with a senzu despite his own stamina loss because he has additional reserves to pull from (though if Cell was mortally wounded and couldn't regenerate like Vegeta was by Krillin, then this wouldn't even matter). But the only reason Goku's punches do anything to Cell is because he wasn't using those reserves yet.
There might be a difference between a 9,000 with no reserves and a 9,000 at half power.In all seriousness, Vegeta's durability is the same as Burter being faster than Jeice or 17 being quicker than he is strong. Vegeta at 18,000 might be more durable than Cui at 18,000, but Vegeta at 9,000 is always half as durable as Vegeta at 18,000. It's spelled out to us that this is exactly how it works.