8606 for prior years - Amend return or just file 8606?

Situation

- I contributed $7,500 after tax to my IRA on 28-Dec-2023, but did not mention that in my 2023 tax return (i.e. did not include form 8606 for 2023).

- That amount grew to $7,925 by Feb and I converted that to Roth IRA on12-Feb-2024.

- I received 1099-R in 2024 from my brokerage company. Now I need to file 8606 for 2023.

- I know that the 2024 tax deadline has passed and I have filed extension for other reasons. I am trying to file 2024 taxes now.

 

Questions:

1. Can I just mail form 8606 to IRS, or I need to amend whole return (i.e. file 1040X)?

2. Will the delta amount $425 (=7925-7500) be taxable for year 2024?