Can an n-channel mosfet gate be held closed for months at a time and still reliably open when Vin falls below threshold?
Can an n-channel mosfet gate be held closed for months at a time and still reliably open when Vin falls below threshold?

Yes, provided the mosfet never leaves it's safe operation conditions. That is, the current, voltages, power, temperature, etc. never exceed their rated (functional) operating limits.