Hi there, I've searched high and low, but couldn't find something to help me with this, or maybe I couldn't find the right words.

Anyway, is there a way that I can install this pre-decorated terrain map as a subhood/ shopping district in my current active neighbourhood (Belladonna Cove)?
Or will I need to install the terrain only and decorate it myself?

Yes, you can use HoodReplace (see my profile). Alternatively, you can change the decorated map into a subhood and then attach it using the standard subhood logic.

Is it possible to turn a neighbourhood, occupied with lots, into a subhood? Or does HoodReplace only work for terrains?
I looked up your tutorial (How to Create a Custom Subhood Template) but I don't think that'd work for making a neighbourhood that has lots into a subhood, although I know absolutely zilch about this.

HoodReplace will not move lots for you, but plasticbox documented a technique where you can move or copy lots from one neighborhood to another:
http://www.modthesims.info/showpost...86&postcount=47

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You can also convert a neighborhood with lots (and even sims) into a subhood template, as long as the neighborhood does not have any subhoods itself..

I don't have a tutorial, but the technique is basically explained in this thread:
http://www.modthesims.info/showthread.php?t=420196

Note that the above thread explains how to start from scratch, so there are many steps that will be unnecessary. Converting an existing neighborhood is discussed in several of the later posts in the thread.

TYSM, Mootilda! You are so very helpful.