I honestly don’t see yet how setting a high critical level to avoid the repugnant sadistic conclusion would automatically result in counter-intuitive problems with lexicality of a quasi-negative utilitarianism. Why would striking a compromise be less preferable than going all the way to a sadistic conclusion? (for me your example and calculations are still unclear: what is the choice set? What is the distribution of utilities in each possible situation?)
With rigidity I indeed mean having strong requirements on critical levels. Allowing to choose critical levels dependent on the choice set is an example that introduces much more flexibility. But again, I’ll leave it up to everyone to decide for themselves how rigidly they prefer to choose their own critical levels. If you find the choice set dependence of critical levels and relative utilities undesirable, you are allowed to pick your critical level independently from the choice set. That’s fine, but we should accept the freedom of others not to do so.