Does the MCU get bricked/locked?
Yes, it's possible. The chip requires a valid clock to run, even when programming, so if the clock settings are corrupt or were set wrong it will no longer be programmable through ICSP.
How can I recover this?
Depending on what has actually happened you can either supply an external clock signal to the chip to get it ...
You cannot. The TX and RX lines are UART. UART is not USB. USB is a very complicated protocol, and you need a proper understanding of how it works. It's not a simple serial protocol like UART.
Add to that the fact that the Arduino can in no way handle the amount of data involved in video, and you're already on a non-starter.
Your initial assumption "An ATmega microcontroller can be flashed by a C compiler and a USB dedicated device (ISP)." is wrong.
The C compiler converts the source code into assembly language. It is then passed to an assembler which converts it to object code. The linker then creates an ELF file from it all. That is then converted into a HEX file using ...
Look at the following pinout diagram (from telecnatron.com):
The right-hand diagram above shows the USBASP-board pin-header pin assignments, when you are looking into the pin-header. The header shell will have a triangle marking pin 1. Note that pin 1 is on the same side of the connector as the key (the ridge on the cable connectors, the slot on the pin-...