We'd need to know the telescope you are using to make any suggestion - are you using the <6" F/5.9 TS Achromat>..?
Is your mount the <Hand Made Alt-Az Mount> also in your signature..?
Someone with a similar bit of equipment would need to help you if that is what you are using - for instance our C14's information would be of very little use to you!
But for your a) there is no "setting" as such for RGB or L in FireCapture...many newcomers make that mistake because FC gives you options for different filters you might have & choose to use with each & any planet.
But any default values in each of the controls (exposure, gain etc) that might appear when you first select "L" or any other option is not to be presumed to be an advised setting - they are simply arbitrary values that exist in the software until you make your own selections using each of the individual controls.
Once you decide what values you decide to employ for any specific planet & filter & then run even a very short capture then FC will remember them for the next capture, unless you alter them & run another capture at the new settings...
You can even do some indoor or outdoor testing with the camera & regardless of what planet you designate a capture as, as soon as you run a capture that will be what appears as that planet & filter's settings the next time you do so. (you could be looking at the view in the distance with your scope & camera & call the view "Mars")
But as a very rough guide aim for a histogram of around 50-60% which you obtain by a combination of "exposure" & "gain" amounts - exposure is really the control that sets the frame-rate (fps) & gain is adjusted to get a specific histogram value at that exposure setting...
There are numerous tutorials for people starting out & I believe that "WarmWeatherGuy" has a very good one for beginners!