Hi Spot
It looks like you have a handle on the 2020 stuff.
For 2019, technically you are supposed to report a change in expected income and would thus be bumped off the silver plan and pushed to medicaid. If you don't do this, there is no penalty or repercussions that I am aware of. At tax time, you might have to repay up to $600 of the PTC (PTC repayment is limited.) This clawback may not be enforced at low income, but TBD.
Since you will be enrolling for 2020 ACA coverage in Nov 2019 and not filing taxes until April 2020, you will just self-report expected 2020 income and enrollment will go smoothly. Most likely. If proof of income is requested and you can't provide it, you will probably be enrolled in Medicare until you can do so. Sounds like that won't be an issue.
One option you could pursue (if possible) is to do a Roth conversion or harvest capital gains before end of year to bump income for 2019 to 138%+ FPL. This would eliminate the gray area as you fully qualify for ACA and PTC at that point. This would mean zero PTC clawback and a good long-term financial choice in any case for a low-income year. (Zero FL income tax, no ACA subsidy penalty, and no Fed tax for MFJ with <$24k income.)
Disclaimer: I've not been through this myself so this is just how I read things. Note the regular usage of words like probably and maybe.