I haven't read every post but just to add a few things:
I have had several of these tanks on order since August of 2021, nothing yet, don't know why this is such a hard to get part.
We tried an ebay/amazon chinese special to get a customer going, but it did not work, alarm still going. So we bypassed him in the meantime.
And lastly, the oil purge with the Mercury G3 software is important. As a guy who didn't get started all that long ago wrenching on boats, and learning along the way, I greatly appreciate a man trying to save a buck and work on his own rig, but this is one of many many jobs that we see quite commonly performed incorrectly, and costs the customer in the long run. So, you guys just do your homework and make sure you have a good shop you can trust to perform stuff for you when you need it and it's something more technical.

Is this oil tank supply problem limited to Mercury outboards or does Yamaha, Honda, Suzuki have similar issues with vital parts? Are new Mercury 250's (as well as other brands 250's) being manufactured or are they also without required parts? And Biden pissed off China....gotta love that guy.

Still wondering if other outboard manufacturers have obtaining parts a major problem? I haven't heard of others besides Mercury...

There are problems across the board. Yamaha wouldn't set us up as a dealer (short version of story, there's a little more to it) because they were 18 months from being able to actually get us some inventory. Tohatsu had issues getting a us a drive shaft last month. I have an Evinrude diaphragm pump I ordered 6 months ago still not in. I could go on and on and on. It's just odd stuff here and there. And new motors in general.