Everything you listed has been done to death. IMO, pay for a membership if you plan to keep your K5 and utilize this site. THere is a wealth of info on here that will answer 99% of your questions. Im not trying to be funny, but the search feature has saved me alot of $$$$$$$ by simply searching and finding the answer so I can Do it myself or found it I had a bad idea cuz others have BTDT>
The rubber matting is fine. I had LMC rubber mat after I pulled my carpet. The mat is heavy and thick and is a PITA to put down. I never was happy with it, so I yanked it out and had my interior LINE Xd , best thing I did. no muss , no fuss. I dont have to worry about anything on the floor or what not.
As far as the dash, I have no idea
As far as the diamond plate, that has been done and it actually looks pretty good. But if you use thick plate, you may need some horizontal supports. THe factory panel is just coverd cardboard. With plating, you can put speakers, invertors or build a door and utilize the space between panel and bedwall, good place to stash stuff.
I dumped my panels after line Xing and it looks way better without the panels and frees up that much more room. I lost my speaker location in back, but will remedy that once I get a cage or roll bar put in.