Anyone have any recommendations on where I can get size 45 Merrills (or similar decent make) gore-tex hiking shoes? There's another thread recommending some places on Qing Nian Street and Ren Min Xi.. I've tried all these places with no luck - they only go up to 44.

Can't believe how hard it is to find size 45 (UK size 10) shoes here, it's not exactly freakishly large!

Also, I was wondering whether the shoes you buy in a decent looking shop are likely to be fake. I don't want to end up spending a lot of money on a fake brand! Do I need to be wary?

Welcome to China, where shops tend to only stock what sells well. UK 10 is big over hear.
If you want a better chance of buying original shoes, hit the department stores, but you will pay double European prices.
If you mean Merrell (when you say Merrill), I don't think you will find Merrils as these are branded.
You may find some larger sizes on Taobao. Other people I know have family ship then from home, along with reliable condoms that fit.

I wear a US 12, and I have yet to find a single pair in all of China. I would welcome any recommendations!

In Shanghai, and some other cities is a French sports chain by the name of Decathlon.
Decathlon stock larger sizes up to US 12 at least. These are mostly in sports shoes. This includes some walking boots/sandals, deck shoes, and riding boots (the ankle length riding boots are fine and almost like dress shoes in appearance).
You can get shoes made, but they often look utilitarian and cost 1500rmb or more.
Some get shoes shipped from home, and rely on insoles to make up any slackness. Others buy in Hong Kong, whilst doing a visa run. I tend to buy a few pairs when I am home and carry them back.

ED
There appears to be hyperlinks popping up in my posts. I did not put them there. It may be malware on my computer or on your system