they could easily develop a tool for unreliable speed situations based on inertial so that at least we can level off and maintain an altitude so we can troubleshoot the faulty indication/s.
That tool already exists: about 2.5° ANU and 77% N1 (Airbus twinjet FBW at CRZ lvl). Then you have all the time you need to try to understand what it is going on in the cockpit...
Disregarding/rejecting the erroneous indicated data are the most difficult and resource consuming parts here. And I am not talking about troubleshooting yet but just about the mental process involved and required by the pilot to decide he now needs to set the a/c in this safe configuration before doing anything else!
IMHO, procedure and training are specifically lacking here (i.e. immediate actions in case of unreliable data (speed and/or altitude) at CRZ level).