the situation:
well-water goes to a well tank.
copper tubing from well tank has a branch that goes to exterior household frost-free spigots, and bypasses the water filter and water softener systems, but can take on filtered/softened hot water via a mixing valve.
the well-water has a high mineral content (fe and magnesium or manganese).
the water temps will vary from 50 degrees f (well-water temp) to 110 degrees f (makes washing dogs/car in the winter more tolerable).
there may be a occasional grain of sand in the well-water, which may adversely affect the function of the check valve and/or ball valve.
pressures will be 40-70psi.  flow will be between 0 and 12gpm. 

i plan to install a stainless steel check valve and a stainless steel ball valve in the line going to the exterior spigots. 
the ‘full port’ diameter of the check valve and the ball valve will be the same, either both 3/4″ or both 1″.

two issues:
1.  i can’t find 3/4″ stainless steel check valves with the proper ‘guts’ that are more suitable for for our well-water (unless i multiply the price 10 fold), but i can find the desired ‘guts’ in a 1″ stainless steel check valve.  so the system would look something like the attached photo.
2.  i have heard that dielectric couplers are notorious for developing leaks as the washers in them degrade.  i have also heard that the galvanic properties of copper and stainless steel are close enough that some plumbers think dielectric couplers aren’t necessary (the dielectric coupler would trade one problem – galvanic corrosion – for another problem – degradation of the coupler and leaking).

see the attached diagram. 
there will be a stainless steel close-quarter nipple between the check valve and the ball valve.
the check valve and ball valve will have both ends with fnpt.
if the answer to question #3 is to just stick with 3/4″ check valves and ball valve, then the 3/4″ to 1″ adapters won’t be needed.

questions:
1.  between the copper tubing and the stainless steel check valve at one end, and between the copper tubing and the stainless steel ball valve at the other end, do i really need a dielectric coupler, or do you just recommend direct connection between the copper and the stainless steel?  or maybe even a brass nipple intermediary?
2.  it’s been 45 years since i took water flow physics in college.  perhaps you can help me… what will be the flow dynamics, resistance/friction, pressure changes if i put a 3/4″ to 1″ adapter at each end of the 3/4″ copper tubing and insert a 1″ check valve and 1″ ball valve in the center section?  or in my residential setting would it be insignificant?
3.  or should i compromise on the quality of the poppet and seals (delrin and buna-n) on the check valve, and just get the 3/4″ check valve and 3/4″ ball valve (viton and nylon or noryl)?

any plumbing engineers out there willing to give advice?