There is a lot to unpick.  But Thorn's longer frames also have a different head tube angle and fork rake. These things effect the steering whatever handlebars you put on. And of course head tube angle will effect the virtual top tube length with length of steerer.  Then there's weight distribution, you could have any bar and the same riding position, but those choosing drop bars might also choose a more sporty position, which puts more weight forward.  That won't physically effect the steering geometry, but it will give the bike a different feel (No, I'm not going to define that!) I've swapped a few bikes between bar styles, sometimes it's taken a bit of trial and error, only one time I've given up before getting it right. 
How much any of it matters is subjective. My experience generally is that if I get the saddle to pedals relationship spot on, the handlebars are less important, somewhere to rest my hands in a comfortable position. For myself, that's quite a range, others may require it to be precise. Plenty of manufacturers supply the same frame with flat and drop bars, many riders accept this without any thought and find it just works. Then there's the variations between bars, there is no standard Flat bar and Drop bar, there's probably as much variation between them as within.  There is no aerodynamic advantage to either, you don't see drop bars used in time trials, in the drops is usually too wide to be the best aerodynamically. People like the bars they're used to, or that they like the look of, or that their mates/group ride... I often see riders that I think would be better off on a different bike, but it's none of my business.  I do have to bite my lip when I see someone with aerodynamic gear, sitting up in the wind, when bending their elbows is free and far more effective.
For the OP, there is no short cut, there's no reason it can't work, but no guarantee you'll like it, you have to take what you know about your riding, have an informed guess at what might work, suck it and see.

First thing I'd try is riding it with the screw removed, bumpier the road the better.
If that doesn't work, then with extreme caution and at your own risk:
Thread the screw in from the other side.  Put a coin, or some other flat metal object, between the two lugs. Once it's tight enough to hold the coin, an extra half a turn. Wrap some tape around the slot so it's sealed. Spray With Plus Gas (Other penetrating oils are available, but they're not all equal), leave for 24 hours. Tap it down with a rubber mallet, don't whack it, twenty or so taps, you're only trying to break the grip, a few taps at 45 degrees won't hurt.. If it moves, add more Plus Gas, wait, and try pulling it out, twisting, don't force it. more plus gas if it's moved but not free.  If it doesn't move, another half a turn of the screw and repeat. Don't overdo the screw, if there's a gap between tube and seatpost, there is no advantage increasing it. Plus Gas works, you just have to get it where needed and give it time.

I would recommend some ergon grips where your thumb rests on the top of the grip - not underneath. It makes arthritic thumbs more comfortable. There is a thumb grip on top of the grip for resting against.

Just an aside. I've changed my Brooks B17 ( with some reluctance ) for a Sella SMP TRK ( size large although I'm not) I changed it because the Sella has a cut out and drooped nose; much easier on the pudental nerve. In doing so I lose a little 'thigh control' but I can live with that.

I built a derailleur bike with drops and definitely feel slightly happier when fighting headwinds. My Rohloff bike has been gathering dust pending a bar conversion. ... another sticking point in the conversion is that I now would also want hydraulic brakes.

The best hydraulic pedal bike brakes, in my opinion of course, are the Magura Rim Hydraulic Brakes, among other reasons because they clamp the largest possible disc, the rim of the wheel, so the discs run pretty cool even in urban rush our traffic.

But the rim hydraulics are fully self-contained from handle through tube to the caliper bodies, oil included, service-free for life (another superb feature); however, you need to be brave to try modifying them, because if you cause a leak, your investment is toast.*

I'm talking here about the HSxx series, of which I have experience of the most junior member, the HS11 deliberately chosen precisely because it provides less clamping force to begin with, and therefore less sudden, more controllable stops while for practical purposes the clamping force is well able to stop even a heavily loaded bike and heavy rider in an amazingly short distance without any of the drama of locked brakes that would accompany the maxed-out brakes the cafe racers brag about.

Unfortunately today the HS11 and the HS33 have the same amount of fluid in them to be compressed (which corresponds roughly to the size of the chamber in the more common type of wheel centre hydraulic brakes) and the only difference between them is that the cheaper -11 has attractive colored trim covers available while the -33 is monocoloured a nasty grey, supposed to look 'professional'. I also vaguely remember different handle lengths at least theoretically available to make fitting on busy handlebars easier.

The last time I looked, because some idiot opined that real bicyclists fit the racing versions, there was an HS22, available to fit drop handlebars, and there was a real racing version above the others which very likely fitted only on drops.

The racing versions come  with progressively smaller chambers and thus higher clamping pressures, at the cost of less nuanced control capability. If you choose the Rim Hydraulics, and have to take a racing version to fit drops, learn to brake with one finger, like the MotoGP racers atop 200bhp machines, approaching a tight hairpin at 230mph.


One more consideration. Though I'm a hard but not harsh braker at the T junctions at the bottom of steep hills, I don't brake often and I'm certainly not a comfort braker. Besides, I ride on general clean tarmac lanes. Despite all that, which you would think should give me impressive rim mileage, with only 11K plus kilometers on the rims, there is visible wear. Either my rims are soft aluminum, or the HS11 Maguras are still too much brakes for my environment and riding style, or Magura's own brake blocks, which do last a long time, are unnecessarily abrasive. Though not all that important to me except for hating waste, for a commuter that could easily become a cost consideration.

*I bought the biggest Service Kit, which is actually a total rebuilding kit, consisting of enough oil, tubing, olives (they fit inside the junctions tube to metal parts), whatever is necessary to fully rebuild one brake, because I knew I would be moving the handlebar up quite a long way even on a 59cm virtual top tube bike I had bought sight unseen, and feared the standard tube wouldn't be long enough, but it turned out to be just long enough, so the kit is unused. Today I wouldn't buy the expensive service/rebuilding kit, I'd just get a competent specialist to supply a new brake set on which he had already extended the tube length.