STM32 Bootloader config. and datasheet discrepency
Q1) Correct. You can use SWD/JTAG directly to download your firmware to chip. Or you can trigger the bootloader and use USART and other available interfaces to download the firmware.

Q2) No, bootloader settings cannot be modified. It only uses the pins it uses, you cannot and don't need to configure the bootloader operation.

Q3) It is possible that there is a typo, it would not be the first typo in STM32 documentation. The bootloader appnote section is for both L01 and L02 chips, so it might not fully apply to your specific L01 chip.

Q4) On your chip, it only uses USART2, but it can look at GPIOs at both PA9/PA10 and PA2/PA3 pins simultanously for activity, and then route the USART2 to the pins that had activity first, so it will only use the USART2 peripheral on both sets of pins. So it really does not matter for the user how it works and how many USARTs there is, as long as you can go to bootloader and use either PA2&amp;3 or PA9&amp;10 to program it.