By "long rim brakes" I assume you mean calipers, which I'm also guessing means a drop bar? I never understood these brakes on a Mercury, I see the calliper mounts have disappeared from the current model frame.
There is a third way and that is to fit a V brake fork. V brakes work fine with drop bar levers, as long as you use the appropriate lever. You'd have the option on another 853 fork, or the touring fork. I have the former and usually run 35mm Supreme tyre, though it currently has a 40mm Almotion, though they both come up a bit smaller on my rim. 55mm mudguards and plenty of clearance.
I don't dislike discs (A mental count - I have 5 disc wheels Vs 3 rim brake) but there is a price and that's most noticeable on the front and can be mitigated with lower pressure tyres, which of course means wider. I haven't had a Thorn Disc fork, though have no reason to suspect it's radically different to those I have had. With a disc brake touring bike that took a maximum 32mm tyre, I fond it uncomfortable on anything much worse than smooth tarmac. On a 29'er with 50mm tyres and a very stiff fork, I have no comfort issues on any surface I'm likely to ride.
One thing missing from your question is the type of riding you do. I also have a touring bike set up for audax, this has a comfortable 531 fork and canti brakes, although I could fit wider, I'm happy running 28's on this as it rarely goes far beyond the Tarmac.
It seems to me, that you can quite cheaply work out whether a 28 is going to suit you, just get one. I like the Continental GP4S, but there's plenty of choice. If it didn't, I'd replace with a V fork, whichever the budget allowed for. I'd only opt for the disc if I was going to use the maximum tyre size that fits.
If you want to experiment, consider a non Thorn fork, depending on which you have, the size and offset might not be hard to find.
