The 'no mixing sprinkler styes rule' doesn't have much to do with pressure, its about application rate and not having one section soggy while the other section is still too dry. Most drip sysems can handle 50 psi, if you're going to mix the zone anyway, I wouldnt bother with a pressure reducer unless your using some real cheap drip brand, it won't affect the amount of water coming out of the emmitters. It's easier to mix a zone with rotors than say spray heads. You can usually nozzle your rotors down so that less water comes out, nozzleing them up might not work, depends on how much of your availabe water is being used already. Drip emmitters also come in sizes. Usually .5, 1 and 2 gpm. really some testing is needed. How long do you usually run the lawn zone and how often? You'll need to choose an emitter size to water your shrubs in the same amount of time. There are also other styles of garden emitters that are individually adjustable, often you can shut them right off, but i find most of them put out a lot of water... if I have to mix a spray zone, thats what I typically use. Hope the rambling helped, I can clarify if it didn't!

All residential gas 'mains' are fed from a higher pressure line at the origin of the main. The pressure is lowered by means of an adjustable regulator. If the pressure in your neighborhood is that low others must be expriencing problems also. Ask the utility why they can't increase pressure at the regulator. (The 'Main' I am speaking of is the pipe that runs down your road and all the service pipes to the homes hook to it.)

4 to 8oz in standard residential line pressure

I can't comment on your valve or pressure regulators but my recent experience and research showed me how important pressure regulators are. On my recently installed/re-vamped system I was starting to have problems with 1/2" tubing popping off fittings and two emitters that actuall blew apart! My investigation showed that my 60# water pressure is much too high for drip irrigation equipment; around 25# pounds is appropriate. But, you need to choose the correct regulator because of the low flows involved. The Rainbird L-30 is designed for the low flows common to drip irrigation.