It, cann't. L1 must work. L5 mainly improves the accuracy, positioning mainly depends on L1. Therefore, the single channel can only work on L1.

 Generally speaking, only common precision will be considered to add LNA. Take MS32SN1 and MS31SN1 as an example, they come with the high precision applications and the gain is already enough for high precision active antennas, so there is no need to add more inside the module. For normal precision, it may be paired with a small size antenna, so the LNA is usually built into the module.

It can be turned off by the AT command.

Yes, self-built reference stations also need to be set up while keeping the reference station absolutely stationary. We have host computer software to support the reference station terminal to build the station.  

1%*driving distance, such as tunnel 100m, after the tunnel out, the error may be about 1m.

 The Semtech SX1262 chip solution.

The RTK-enabled modules also support RTD, RTD is also a kind of differential, the difference is that RTK is the carrier phase differential, the carrier wavelength is about 20cm, RTK high accuracy, can do centimeter level. RTD belongs to the pseudo-range differential, pseudo-range principle is based on the propagation time of electromagnetic waves, the accuracy can only be to the meter level, such as about 1m.

The ST STM32WLE5CCU6 chip solution.