Painted door tests are your friend here (google it). You could create two or three landing pages with different message variants, each of which leads to a "request access" form. Depending on what your campaign is for, your message testing could be as simple as running it by product managers or account managers. Or you could grab a few web visitors through a Qualaroo survey and interview them. You could grab people and buy them a coffee at a conference. Basically, there's no big trick to this - you just have to do it. If you're getting feedback on your messaging from your target audience or someone who can reliably act as a proxy for your target audience, you're probably doing it right.
For sales messaging, I haven’t encountered anything better than “Command of the Message” which you can google.
I'll take a more extreme position on this question. You're setting yourself up for failure by asking us how to "defend" your messaging. Instead, I'd ask you to listen to those people who you are used to "defending" your messaging from. It's not your messaging - give up that pride of ownership in order to listen and learn from sales, product, and your executives if they choose to care about your exact phrasing. That doesn't mean that they get to write the exact words - but all of those constituencies have an important point of view, and it's your job to triangulate among them, decide where to give weight, and come up with messaging that is clear and compelling. When you think you totally have that message nailed, you're not finished. You're at step 1. Next, you need to test that messaging with real, live customers. Don't look just for confirmation of your message - lean into those people you interview who find your amazing messaging confusing or bland. Figure out how to make it right. This is an involved process, but it's amazingly powerful. Then, by the way, when someone comes and says "hey that message isn't that great, how about XYZ" you can say "well, we crafted this message with your input and the input of other leaders, then we shared it with customers and made ABC changes based on their feedback, and this is where we landed."
I'm going to take a somewhat contrarian view on this and say that in order to really break through in a crowded market, it takes more than clever messaging (though that never hurts). You have to position your product correctly and you have to prove that you are better. Now would be the time to invest in a really solid customer marketing program to tell their stories of success with your product. All the better if they moved to your product from a competitor. Include that question in every win interview you do, and produce "grey label" case studies for your sales team. Those look like "A major auto manufacturer used to use [competitor] but they switched to us and here's what they have experienced - higher sales, lower costs, lower risk, etc."
Depends how market-facing your PMs are. If they're obsessed with serving customers and creating products and features that will generate new business, probably don't need to be very involved.
If they're focused on internal things though and are more interested in how engineering will implement something vs. why - that's where PMM needs to be very involved
Mike's answer is spot on. Since you've been laid off 5 times in 10 years though, I'll add one other thought. It's possible that you're focusing on outputs, not outcomes. PMM done right is much more focused on outcomes - well defined, measurable (as much as possible). It's the PMM lead's job to secure buy-in from execs, product leadership, and sales leadership on those outcomes. Then it's your job to hit them. It's easy to churn out 2 white papers per quarter and think you're doing something. It's a lot harder to sign up for a web conversion metric or something similarly relevant for your business.
Twilio, splunk, nginx, newrelic are good examples.
To me, a solution is a prescriptive collection of products and features that solve a well-defined problem for your customer. A product is anything you could conceivably sell on its own, but a product can also be a collection of other products. A feature is a component piece of a product that adds to its value but cannot be sold on its own.
Products, features, and solutions tend to get different levels of attention from PMMs. Products will naturally get the most, solutions are really just collections of products and are therefore more an exercise in packaging and pricing. Features get attention insofar as they need to be launched, marketed, and incorporated into the story for the products and solutions they serve. Resource your PMM attention accordingly.
At a previous company, we started by maintaining a wiki page for competitive intel that was the "single point of truth" for the sales team. Mike is correct when he says that at an early stage, CI is everyone's job and the PMM's job to QB the effort. I would suggest that a solid level of competitive intel is necessary in order to truly understand your differentiation and sell effectively. Think about structuring your CI program in terms of structured and unstructured data. Side by side feature comparisons, competitive takeout stories and quotes, and so on need to be codified and hosted somewhere that everyone can access easily. Then you have the more "tribal knowledge" - rumors of something happening at a competitor, fuel for FUD, and so on. Until you are well along in the CI program, it's best to have something like a slack channel but not try to manage that information too closely.
You're trying to figure out why they bought your product, what materials and conversations were most useful in their process, and setting them up for a case study down the line. Questions should go in line with those, and the interview should usually take under an hour unless they're talking your ear off. Dig deep for things that can be improved. Don't bring sales along for the ride!