1.2. How do temperature, humidity etc. affect the laser frequency?

Delta refractive index / Delta temperature (at 780nm, 35%, 100kPa): -9.07e-7/K

Delta refractive index / Delta pressure (at 780nm, 25°C, 35%): 2.62533E-6/kPa
Delta refractive index / Delta humidity (ar 780nm, 25°C, 100kPa): -1.14179E-8/%

For the DL 100 that comprises a resonator with a 20.8mm air gap this leads to the following frequency changes with respect to "standard conditions" (780nm, 25°C, 100kPa, 35%):

D frequency / D temperature: +218MHz/K
D frequency / D pressure: -631MHz/kPa
D frequency / D relative humidity: +2,7MHz/%

Typical fluctuations over the course of one day in a lab could be:

f Temperature / day: 2K result in  a frequenzy drift of +440MHz (this does not include frequency shifts by mechanical effects but only the air temperature)
f pressure / day: 0.2kPa (mean variation over day) results in a frequenzy drift of -130MHz, large variation of up to 1kPa are possible as well -> drift of -630MHz (!!)
f relative humidity / day: 5% result in  a frequenzy drift of +14MHz