As always, Sheldon (R.I.P) has the answers:
https://www.sheldonbrown.com/stuck-seatposts.html

I fought a 3 week long battle with a secondhand Fuji Track frame and seatpost a few years ago, I tried WD40, Plus-Gas, gearbox fluid and acetone mix, clamping the seatpost in a bench vice and turning the frame, then tried dissolving the seatpost with caustic soda (aluminium post in a steel frame) with no joy. Eventually I bolted 2 hacksaw blades together, cut the top of the post off and after several slow and careful sessions managed to cut the post into 4 strips from the inside, grip them with mole-grips and twist/pull them out. I now have the remains mounted on a plaque in my garage to remind me that greasing seatposts is part of regular maintenance.

You mention trying to get more leverage but not what leverage tricks you've tried yet. If you have an old scrap saddle try putting this on the post, you can then hit it, twist it, put it in a vice etc without worrying about damaging your good saddle.

Personally if I couldn't shift a seatpost after a few days soaking and some leverage, and the frame was a nice one that I didn't want to risk damaging, then I would now send the frame off to a specialist. There are companies out there with a pulling machine that will safely remove it for £80.

As B Cereus noted, a hair dryer can be a good source of heat.

I would certainly trust using it for several minutes at a time, but not for extended time.

A hair dryer is designed to be used for a few minutes at a time.  Leaving a hair dryer on for much longer could damage the hair dryer.  I set a hair dryer on something to provide a continuous steady source of low level heat at a low setting, after about 10 or 15 minutes the hair dryer quit running, and never ran again.

I used a hair dryer to warm up my frame bottom bracket shell before I used significant force to remove a stuck bottom bracket that I wanted to remove from the frame.  I can't say if the heat helped or not, but it certainly did not hurt.

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.