For the second bug, we should just be more lenient with our parsing of the era name

Yep, easy workaround. I'll PR that into the polyfill when I have a chance.

I think what the polyfill needs is a map from English month name strings to numbers.

What I'm not sure about is whether there's enough information in the names. The numbers (if I remember correctly; need to check the code to be sure) tell you the month index in that particular year. The names could be different indexes depending on whether it's a leap year or not. The problem is still solveable (first need to determine if it's a leap year, then interpret the names differently depending on leap or not) but it's more than just simply mapping names to numbers.

Also, this CLDR change breaks any other apps that are relying on it to know the month index (or were simply relying on 'numeric' to be numeric) so IMHO I'm not sure this is a wise change to make in the platform.

very brittle to rely on formatToParts output

Yep. Glad that Temporal (polyfill or native) can be used instead of relying on Intl hacks to get Hebrew month indexes.