Here is my take, assuming the husband and wife companies A & B are a control group:
1) Yes, it's plan assets from all plans of the employer/control group.
2) Plan covers only owners and spouses, so yes, EZ still appropriate for now. In 2024, under new rules they may no longer be a control group and I do not think you can file an EZ for a multiple employer plan, which you would then have.
3) No other schedules or attachments.
4) Filing and extension should not trigger anything because the extension would be the first filing of any kind for the plan, so neither IRS nor DOL would have any knowledge of the plan's prior existence or when its assets exceeded $250,000. So I would file the extension, then get 2021 filed under EZ delinquent filing program and then filed 2022 extended return.
Note, if the companies are not a control group - not in community property state, no minor children, no involvement in the other's business - then you have a multiple employer plan and all bets are off and you likely have many more delinquent returns (SFs) to address.
Come 12/31/2023, you may want to spin off B into its own separate plan if/when the control group goes away. That way each has their own $250,000 threshold and EZ filing requirement. Assuming you can show less than $250,000 as an ending balance for A at 12/31/2023 and 12/31/2024 it shouldn't trigger a letter for a delinquent 2024 filing.