What's the recovery plan for a bad 6.14/2015.14 catalog?

I have hit the end of my rope with a 4.5Gb and 249,000-item catalog that chronically crashes. I have tried LR repairs, SQlite repair tools, importing from that catalog to another, accessing a copy from another user account (on my Mac), moving the catalog, deleting the previews, everything. 

It is just cashed out. I get a ten second window to do something other than generate previews. The only thing I can do without crashing this catalog is export or save metadata. I have both RAW files and scanned TIFFs/JPGs. How do I get as much as possible into a new "add in place" catalog? Is there a guide to this? I would like to move:

- Exposure adjustments

- Cropping

- Face detection (even if just zones)

- Flags and color codes

- Collections

If plugging into a Windows machine, accessing the same catalog, and exporting as a catalog would do it, I can try that. But I suspect I will still lose a ton of metadata and changes that way.

I am not afraid of using plugins or doing transformations on XML. I am primarily frustrated because there is no documentation telling me what is where (catalog? sidecar? file metadata?). I would think that a sidecar should be everything. But the program does not seem to be setup that way.

And so far it has been taking days to find out what won't work.

Has anyone else been in this boat?

Thanks!