I was just responding to your email, so will post here instead.
First, I apologize for not responding sooner, I was out sick for two weeks, then returned last week to teach a group workshop, so have unfortunately been away from email for several weeks.
Secondly, I am very sorry you experienced a leaking cartridge, especially after over a year of good results!
You said the LM was last working the same morning/day you noticed the cartridge was leaking, then installed a flush cart until the replacement cart arrived. In my experience, the printer will give a pressurization error if it detects a loss of pressure in the ink system. I'm not sure why/how your printer continued printing while the LM cartridge was leaking. From the photos you sent, the cartridge didn't completely drain (it was still about 1/4 full), and it seems to have been leaking from the middle/rear lower side edge (not the front end that is far in the printer), so I don't understand how the ink line could have been empty (which you said in one of the many emails you sent). Considering the cartridge didn't totally drain, and the pressure loss wasn't enough to prompt the pressure loss error on your printer, it seems strange/unlikely that it would cause damage to your print head... Over the years, there have been a few cartridges leak in printers, which is messy and frustrating, but others' have been fixed by installing a new cartridge and doing a few cleaning cycles, or printing color purge sheets if there was air in the ink line. BUT- I have seen and heard of all sorts of strange things happening with the 7890/9890, 4900 and 7900/9900 printer models...
If ink has filled the LM ink line after the last initial fill cycle you did, then ink is flowing thru the line/damper/head channel, meaning you don't have a severe clog (so, I wouldn't suggest flushing the head), but if nothing prints from that channel when printing a nozzle check or image, then that sounds electrical.
Chinese print heads can be perfectly fine (or crap), so it's good that you're carefully checking the warranty.
I would love to help you get back to happily printing, but my help is limited with printer repair, other than what I have learned over the years from experience.
I wish you the best, please do keep me posted!