Did you use "Replace with still" for the modified background? Well, I tried a simple test to duplicate if I get the intermittent highlight problem, and guess what, there's no problem whatsoever. All I do is convert your BMP to VOB with Automux. Then use Replace with still with the VOB. I then export the buttons from your modified project and import them into the original project. Then I export, and no intermittent highlight problem. I can also use the arrow key to highlight other buttons. I understand the highlight is not complete because the subpicture highlight needs to be modified as well, so it can be enclosed within the button rectangle markings. Look in the attachment and test for yourself.

So it seems that your version of DVDRMP is not producing the output as I expected. Registered version does not have this problem at all. Please read HERE.

You can do all of this with DVD Styler, I think. No need to integrate with DVDRMP. I think Gui For DVDAuthor is able to do all this. Also, TMPGEnc DVD Author (fully functional 30-day trial) can as well, and is very easy to use for a beginner. These are all-in-one packages, with some limitations. You can't expect a package to do everything a user want. It'll be like bloatware, like Nero. Reason for these suggestions are that you seem reluctant to learn new tools to accomplish the task.