I don't know if this will help you at all, seems like you constantly shoot me down...but I'll try anyway.
My H has a tendancy to automatically go to the negative ANYTIME I want to sit down and talk about something difficult, or talk about an issue that he's uncomfortable with. His defenses automatically go up and he hears me through filters....so he honestly misses what I'm actually saying. Here's an example.
A few months ago I found that he'd been on a boating site that he likes to visit. Now much of what he looks at on that website doesn't bother me....however, some of what he looked at in the past (naked bleached blondes) does bother me....especially considering what I've gone through since Feb. I'm doing much better about this as a trigger, but when I'm triggered, I try to communicate to him why.
So...I found some pictures on our computer. Now being the computer savvy woman that I am, I could tell these weren't photos he'd necessarily clicked on to view (they were thumbnails in a gallery of other photos, I verified that when I went and looked). Anyway....just seeing those thumbnail photos brought back bad feelings for me...so I was trying to let him know WHY I was in a quiet/bad mood. I wasn't attacking him, wasn't even accusing him of doing anything wrong...just saw something that triggered a response in me.
When I tried to talk to him about it he immediately started accusing me of not giving him credit that he'd been looking at other stuff on that site...boats, engines, a dog in a boat etc. I literally told him seven times that "why would I bring that stuff up? I knew he had viewed those things, but those things aren't issues for me....naked women, those are issues." I kid you not, I had to calmly repeat myself to this red-faced man seven times (I finally started counting) before he calmed down enough to actually HEAR me and see I wasn't attaking him. Did I get frustrated? Oh hell yeah! I even flat-out stated to him "I know you were looking at all sorts of stuff on that website, stuff I have NO problem with. At one point he was so angry he wouldn't even sit down next to me to talk, because he was hearing me through his defensive filters. Not until I looked at him with what I know was a confused look on my face and asked him "Why are you getting so angry? I haven't accused you of anything, I haven't said you did anything wrong....I'm just telling you I saw something and it triggered memories/feelings, I just wanted to talk it out so you would know what was going on." Right about that time is when I saw his color returning to normal and I could listerally see the light of reason returning to his eyes. THAT is when he started to hear what I was saying. Yes, it took me repitition and persistence....but above all it took ME staying calm. I have no doubt that had I gotten angry that he wasn't hearing me, or that he was overreacting to what I was saying....he never would have heard what I was saying. (BTW...Mr. GEL is someone who tends to react to the extremes, esp if he feels like someone is attacking him, trying to prove he's wrong, or pointing out flaws.)
Cobra, my H is used to being on the defensive, which I suspect that for whatever reason (even if it's her own doing) your W is too. He automatically assumes that when someone has a differing opinion or POV than his....they are going to try to prove him WRONG. I can't escape this feeling that your W is much the same. So with him I honestly have to take an approach which takes right/wrong out of the equation, it throws him for a loop too. I'm seeing great results though with this approach....but it's taken time, persistence....and hurculean amounts of patience. You asked for a diplomatic approach...there you go.
I feel like often your approach towards your W makes HER feel like you are constantly pointing out what's wrong with her (legitimately or not) and making it a right/wrong issue. She WILL fight you on this...it's in her programming, she's been fighting you on this...she's going to continue to do so unless you change you approach and give your approach time to work.
Is it possible for you to maintain an approach that takes right/wrong out of the equation? Now before you say "no that won't work and here's why..." to that, that does not mean she has to agree to adopt that approach herself (my H NEVER has adopted it)....only YOU have to be willing to try it. It is working for me, and I'm the only one adopting that approach....my H doesn't even know I've recognized this in him and have altered my approach with him. If I told him that....it would change the dynamic and defeat the purpose.