- no, most likely analog signals cannot be read from M5Core2 port A due to the two pull-up resistors.
- no, port A doesn't need to be enabled first. It is already enabled.
- no, analog signals don't need a clock. The clock is used for I2C.
Note: as mentioned before, I don't own an LM35 and cannot test this myself, so my conclusions, solely drawn from reading the schematic, could be incorrect.
M5Stack has defined three Groove ports A, B and C.
- A (red) is by default used for I2C (found on M5Core2)
- B (black) is by default used for DAC / ADC (found on M5GO Bottom2)
- C (blue) is by default used for UART (found on M5GO Bottom2)
There are exceptions to the above where a function can be setup and used on a port not meant for that. In case of port A in M5Core2 there are this two pull-up resistors already in place (which are required for I2C to function) but they also might or might not influence other usage.
Given that in your case you are reading the same value independent of the actual temperature, I assume that yes, the pull-up resistor messes with the analog signal from the LM35 and therefore port A most likely cannot be used for the LM35.
Your best bet would probably be to get an M5GO Bottom2 and then use port B (ADC) for the LM35. Or you could get an EnvIII Sensor (temperature and humidity air pressure) which uses I2C and works on port A.