If you come into my house in the middle of the night, I am fully justified in confronting you with my shotgun. I'm not sure how that takes diplomacy off the table. How about: "Wait! Don't shoot, we're here to fix the phone line!" That might make me click the safety back on. Or it might not, but the option for diplomacy remains.
I really like that analogy. I can sympathize with the OPs DM about roleplaying before combat, but I can also sympathize with the OP about the players not thinking about roleplaying before the combat starts.
I've found myself criticizing the players for not being diplomatic and instead going straight to combat. But then I realize it's a game where players are used to "murdering" tavern patrons during a bar fight where the PCs threw the first punch because the civilian mouthed off to him and made him look stupid. The combat is the fun part and is what we resort to quickly because it's exciting. So many times have my players killed first and then thought, "Crap, maybe we shoulda asked some questions first". Sometimes as a player you just don't think about it when the DM describes the BBEG as looking ready for a fight. So I've learned to be understanding towards my players and I realize that I need to put forth that extra effort to "nudge" them into roleplaying before the fight. It's not always their fault for killing first and Speaking with Dead later
Hell, it took him two adventures to scale up our armor to a manageable level; I had only a +1 suit of armor at 9th level (that I found at 2nd level during Keep on the Shadowfell) and then it got "upgraded" to +3 because I guess he realized our defenses were horrible.
Really, you should ease up on the guy. This is a completely common thing about running a game. The DM is but one man. He has an entire campaign to run & prepare for while you have a single character to look after. Being a DM yourself, you should be understandable that it's pretty difficult to keep up with every single PCs power level and character wealth...especially as they level up every time and these things change. Personally, I'd rather my DM bone up on making a cool adventure and providing some exciting gameplay opportunities rather than focusing on my character wealth & power level. If I notice a problem during encounters, then I feel it's up to me to let him know so he can prepare for it later when he gets a chance.
So lets see, you were underpowered due to lack of appropriate gear, the DM realizes this, and then gives you your much needed upgrade. And the problem is that he didn't do this fast enough for you? Were there character deaths due to the fact that you were under equipped? Did you not overcome an encounter because you were under equipped? If either of these were issues for your group, then I can understand your gripe. But if there were character deaths because of this, and nobody pointed it out to the DM, then I can't sympathize with you. But to complain just because you didn't get your goodies fast enough just sounds silly to me.
Reading the module is just the equivalent of reading the walkthrough to a video game.
The difference is, a video game isn't going to care if you're cheating
Nobody is saying that they are going to tell their DM that they are going to read the module so they can metagame. What they are saying is that they are going to flat out cheat. It doesn't matter what people say to justify it, it's still cheating and pretty childish. But if they are up front with their DM and the DM doesn't care, then I see nothing wrong with the cheating.