posting this as it may help someone else.
I have been working on a full length doc for 3 years, and as you never upgrade in the middle of a project, have been stuck with a FCPX version with memory leak and other joyous bugs, and Mavericks, which has some bugs of its own. combine that with 3rd party plugin bugs and corrupt clips, and you have a deep level of hell. the doc is complete and the timeline very complex with subtitles, masking, CC, grading and effects on every one of 100s of clips. trouble is, it will not output. FCPX crashes, computer crashes, plugins crash, and MBL gets stuck in a deauthorization loops with flashing red Xs. the crashes happen at random times for different reasons in the export, and often after 7 hours into the export. weeks of this with no real solutions. resorted to exporting small pieces, but they were often flawed, and it seems a bad workflow to have to reassemble and export the whole thing again, and i would imagine take a quality hit (it will also be output in 5 languages...!)
anyway, during many weeks of detective work, i consistently read that rendering the timeline is not necessary, as it will re-render on export anyway. Well, I spent about 9 hours rendering (with many crashes and restarts) and, lo and behold, the entire 90 minute film then exported in about 10 minutes.
So long story short, if exporting is not working, render before export! if there is a crash while rendering, you don't lose anything. if it crashes on export, you lose all...

Back everything up, project, system, everything. Wipe your computer and reinstall the system fresh and programs and personalizations from the backup. Your problems are absolutely not typical and lead me to believe that there is something very wrong. Once an effect is rendered, FCPX merely copies the render file to a new ProRes master without any re-rendering.

I had export problems way back with FCP7 where the only way to isolate the crash was to render everything in the timeline in small batches until I found the culprit. The only time this has ever happened with FCPX was with extremely large TIF files that worked live in the timeline but wouldn't export to a file. Resizing the TIFFs fixed the problem.

That is exactly what I will do.
but first i need to meet my deadlines, and by rendering, it seems i might!

it seems the last version of yosemite may keep all my software working without paid upgrades or becoming a feudal software serf (which i will resist as long as humanly possible).
trouble is, Apple does not make it available. what the??!
what kind of company punishes loyal customers by limiting their options, and with what logic?
as a video/audio/photo/graphic professional juggling all sorts of hardware and software, i have to have a stable system and can't constantly update and deal with bugs all the time. i need my system to be reliable, so i do major updates/upgrades every few years, or when absolutely unavoidable. and it is inevitably a week of pain and detective work.
so what options do i have to obtain yosemite? do i physically have to go to apple store and have it done?

Try this out:
https://discussions.apple.com/thread/6729003?start=0
According to various sources, you should be able to download it from the App store program in the purchases tab.

Here is the updater if you never did it: https://support.apple.com/kb/DL1832?locale=en_US

Have you explored using the "cmd R" Internet Recovery option?

There are a few places that claim to have the dmg file for a full install of the Yosemite system but I am a little leery of them.

You should really explore updating. The bugs you are worried about are not there, at least in my experience. A few old filters will require updating for El Capitan. However, I did keep one old computer in Yosemite because of an important program that I use hasn't been updated by the developer (and it may never be).

thanks for the info!
just starting to research options, and yes, my Pro Tools and Photoshop would likely die a buggy death with el capitan. there is no option in purchases for yoemite that i can see, and the sites that provide yosemite downloads look pretty sketchy indeed. need to try Internet recovery, i suppose...

dead end after dead end. called apple care and they are unhelpful. i was actually told to go find someone who has it and download it from them. whaaaaat???
this is pretty outrageous. what possible reason could they have for withholding or making an older OS unavailable? i feel like i'm being treated like a bad little kid, not a professional studio owner.
apple seems pretty rotten.

had to go to apple store to get yosemite clean installed. they would not give me a copy, or put it in my app account. wow.
so i wiped my computer after cloning, clean installed yosemite 10.10.5, did clean installs of all my programs and plugins with latest possible updates. repaired permissions at each install. a lot of effort and time.
results:
FCPX is slower than before, many of the bugs remain, and now some new ones. the timeline curser has graphics tearing, importing files is slow, even when the files are left in place and are one frame long, export is slow, even to get it started, and even when exporting one frame, and lots of spinning wheels of death. the older version is faster, before the memory leak inevitably kicks in....
the film has a few sorcerers in it. maybe they cursed me?

Nope, something is wrong here outside of Yosemite and FCPX. Wish I could suggest what.