I can't help with the Nautolex, but I have some experience with the t-nuts.
I mounted mine the exact same way as yours. And that turned out to be a mistake.
Months later, while installing the seat bolts, several of the t-nuts pulled free of the plywood underdeck backing plate and fell into the nether lands below the completed deck. Even though they were glued in place. I ended up having to find a way to fix the issue, which I did. Took me a long time and plenty of research. I also had to pay for some expensive bolts, but I got it done.
In your case, if you can, I'd remove the t-nuts from their current location. Then I'd pull the backing plate and sandwich the t-nuts between the main deck section and the backing plate, then reinstall the backing plate. That way, the t-nuts can't ever fall out.
Just a thought. Gained from painful experience.