Heather,
I feel your pain. Some suggestions, more in hope than certainty;
You don't need to care about the middle joint, if you can thin the leading crankpin enough, you can get clearance behind the crosshead. If the middle crankpin is a few tenths outboard of the leading one, nobody will be able to see, and it won't matter a jot from a geometry point of view.
So, I would not modify the centre or trailing crankpins, rods or whatever, but simply use a Slaters bush with flange to the wheel for the coupling rod, and another flange outboard for the connecting rod.
I'd stick with the Premier rods.
How thin can you make the rod where it crosses the leading wheel hub?
I think you've already recessed the rod where the crank-nut fits, I did this on my 1366, and used a Slater's bush, arranged so the flange and bush were fully within the thickness of the rods. DLOS also did this, he made a nicer "more professional" job of it than I did, he machined the holes in the crank nut and made a peg spanner to tighten it. I use filed notches & tweezers!
Can you put an axle in the lathe, and skim the front of the wheel around the hub? If will still need to be proud of the rim, but I guess a few tenths can be taken off there. Youll probably have to deepen the countersink in the wheel to get the screw in deep enough, possibly also the countersink in the axle ends too.
How thin can the flanges on the back of the crosshead be - I'd guess 0.25mm might be tight but do-able?
Can you thin the slide bars sideways a little? A few swipes with the file may gain another tenth or two? But that might suggest that you'd need to make the crosshead slots thinner too, otherwise it will be free to move in and outboard, and that's more difficult.
hope this helps
best
Simon