Situation 2 I think would be okay overall. It makes sense for a mobile champion to opportunistically jump onto weakened target, and if you're at 20% health, it would make sense for you to be more vulnerable. Situation 1 I agree is bad when the enemy champion's mobility feels like a get-out-of-jail-free card, i.e. a piece of mobility that they didn't need to use beforehand, and can use to reliably escape with no real repercussions.
What I'd consider most frustrating, however, is when you're playing an immobile champion at 100% health, and a mobile opponent jumps you and out-duels you by design. At 20% health, it would make sense to be at a disadvantage, but at 100% health, and with no mobility options, there is virtually nothing one can do to avoid combat with a champion capable of engaging on demand via mobility. When there are also few options in combat to out-duel that champion (and this happens often, as many mobile champions are also duelists), the lack of agency there compounds to make it feel like one had no option but to lose to the mobile duelist, simply by existing. The lack of strong defenses to fall back to in League at this point make the problem worse. Because of this, I think Riot is going to need to eventually take a harder stance on mobile champions, and force champions to trade off between target access and required opportunism. A world in which champions can either engage whenever they want, or engage targets reliably, but not both, is one that would be fair for both parties involved. As long as champions continue to exist in a state where, on equal power, they engage both reliably and regardless of opportunity, they will continue to deny agency to champions who can't do both.