Only thing that occurs would be to make those little illustrative symbol discs for Tamran, Fort Ramgate and Lost Mines larger. It seems a shame not to better highlight those little pieces of artwork. And of course to maybe add more for other places too.

That might mean adding dot-markers for the places, and a line, or two lines, linking to the enlarged "art" view for the places, however, so you're still showing their precise locations, as well as illustrating them, though that may be more map-clutter than you'd prefer.

Yes. It maybe there's something in one of the numerous isometric artwork sets available on a download site such as DriveThru RPG that would help, but there are a LOT of those to try to hunt through, and you'd need to convert the artwork to be symbols in CC3+ first, of course.

Top down for the mountains would seem the better fit for what you're trying to emulate. Plus that would save you having to draw more symbols, and just set up a drawing tool with some effects!

One alternative you might try would be not using the RGB Matrix effect, but setting up a simple rectangular polygon that covers all the map on its own, new, sheet, setting the polygon to have a solid red colour, and then adding a Transparency effect to that sheet. If you move the sheet to be below the text, but above the rest of the map sheets, that might work. You'll probably need to keep adjusting the Transparency, and probably also the colour of the polygon (likely somewhere in the reds and oranges range) to get to something you're happy with. Not guaranteed to work ideally, but it should give you more control over the final colour of your text, at least.

Selecting can be problematic, especially when you have two things that directly overlap one another. Fortunately, there are a number of different options you can select by using CC3+ to identify which of the objects you've selected you actually want to do something with. This post, the accompanying short linked video, and the linked additional post, again all by Remy Monsen, will help with this, whatever issues you're having with selection.

There isn't a global search mechanic across all the add-ons you may have installed, mostly because it's advisable to choose a style to map in, and stick with that, since those items will usually work well with one another in terms of the overall map appearance. That doesn't stop you from bringing in symbols or bitmap fills from other mapping styles, but you'll need to identify what those are when doing so first of all. If you want to see what symbols and bitmap fills you have on your system, the easiest way is to use Windows' File Explorer, and navigate to wherever you have your CC3+ system installed (the standard default is in ProgramData under Profantasy, for instance).