Consider them gone although I would think that if they failed then they might just stay open causing no trouble. We are planning on putting the presoak and tire cleaner on a foam gun later. I ordered the needle valves like you use and a mesh strainer. I'm still on the fence about a regulator. It seems like it would allow a better adjustment of the needle valve. How far open are your needle valves? The ones we are replacing are almost fully closed and I would say that if opened to 1/8 of a turn would put out double the water. At least the new ones have a packing nut on the stem so it should stay wherever they are set. Our current ones go crazy when tightening the set screw.
The needle valves I got from KR are not that temperamental. I don't know how many turns out they are. I use a measuring cup and stop watch to set flow, 16-20 oz per minute. When it comes to flow pressure, my incoming supply is at 40 psi. So that pressure works for me with no regulator. I'd think it through before putting a regulator in the mix. For me its just another failure point....I don't have one and I've never had an issue. Are you sure something else isn't dropping pressure and volume? Do you have a pressure gage on the main supply coming in to the ER? It will tell you a lot.
I think the key is making sure you have enough supply volume, pressure is secondary....Just like plumbing a house. If you have a 1/2" copper main supply and 1/2" branch feeds to each sink, commode, and shower, what happens to the person in the shower when someone flushes a commode? But if the main supply trunk is 3/4" with 1/2" feeders, you won't see temperature fluctuations while taking a shower. How do I know this, My house had a 1/2" main supply to all the branches when we bought it...I got tired of all the complaining of getting scalded in the shower, so I changed the main supply to 3/4" and left the branches to 1/2" ….Problem solved...
The weep system is the same. If you feed the system with 1/2" fittings and reduce to 1/4" to each ball valve and needle valves like I did, you won't see much pressure variation from bay to bay if someone tries to wash with weep (trigger Pulled). I have never experienced what Mep is describing, but I can see it happening if the complete system isn't plumbed/sized correctly.
BTW, my weep is plumbed off the main supply coming in to the wash, ahead of the softener. I can't see any reason to weep soft water! Something else for you to look at....Where is the weep supplied from? Could anything be dropping pressure on that branch or feed? Maybe putting a pressure gage inline on the weep branch might tell you something?