Backfeeding MCU gate output bad?
Is my solution reliable in the long term?

No.
A typical microcontroller IO pin has what is called three state logic or tri state:

The pin can be in 3 states:

Input, with the output driver transistors disabled, the pin is high impedance.
Output low, the top output driver transistor is off and the bottom one is sinking current.
Output high, the bottom output driver transistor is off and the top one is sourcing current.

In your circuit, posted above, it is safe to assume the pin is always configured as an output, either low (MOSFET/LED off) or high (MOSFET/LED on). So it is always either sinking or sourcing current.
When you close the switch on your diagram when the microcontroller output is low, current will flow trough the 100 ohm resistor, trough the bottom output driver transistor to ground. The only thing limiting current is the 100 ohm resistor so that bottom transistor has to sink 36mA. This is much more then a typical microcontroller pin is rated for and can damage it.