Guard schedules are highly dependent on unit and mission. Some units might have a minimum number of available days they want you to provide for the scheduling shop. The bottom line is you need to stay current and proficient. You would need to speak with someone in the unit for get an idea of typical days per month required to do that. After that, it's up to you how many additional days you keep on your airline schedule. You may choose to drop airline days one for one with guard days some months and go heavy on both jobs other months.

Managing schedules is very dependent on whether you commute or not. Living in domicile and within driving distance of you guard unit gives you outstanding options and flexibility. If you commute to the airline and live in your guard city but can sit long call reserve from home, you may be able to double dip. Ground duty or a quick local sortie with the guard while you're on reserve with the airline. If you're flying a line at the airline, then you may choose to drop trips and stay local with the guard more frequently rather than do a lot of commuting. Lots of options that are very situation dependent. What you really want to avoid is commuting to both jobs. Living near your guard unit is probably the best option if you can't be near both by living in domicile.

My guess is the 121 jet time (especially if you can upgrade) is going to be better for getting on with a major. Some airlines want turbine time to be in aircraft with GW higher than 12,500. I think that rules out most King Airs but you might know better than I.