Long rear derailleur for 11/32 cassette
 

I recently purchased a Merlin extralight with 7800 dura ace drivetrain. Love the bike, but it is certainly not geared for my mountainous neck of the woods.

What ten speed rear derailleur would be a good choice to go with a 11/32 rear cassette?

New Shimano XT/XTR is going to be 10 speed. I'm sure there will be a long cage of both that will accommodate 11-32 and still work with your Brifters.

Ironically, if you need to install a Shimano mtb rear derailleur to work with your road shifters, , you can use pretty much any Shimano mtb rear derailleur EXCEPT the new "10 speed Dyna Sys" models to work with your road shifters because Shimano has changed the cable pull ratio on these models. Keep in mind, the number of "speeds" is in the shifter, not the derailleur, it's the cable pull ratio that needs to match between shifter and derailleur. Also keep in mind that the cage length only affects chain wrap capacity, not largest cog compatibility.

If yours is not a triple, you may want to consider putting a triple crank on. And a double, you may also want to consider a 34/46 crank, both with a 28 cassette. Small chainrings I think go down to a 24.

Incorrect.
The new 10s "Dyna-Sys" mtn RD's have an incompatible cable pull.
The older 9s RD's will work with brifters.

Small chainrings for triple cranks with 74 mm bolt circles for the granny do go down to 24T. Compact doubles with 110 mm bolt circles will accept a 33T chainring as the minimum and 34T rings are far more common.

Installing a 9-speed XT or XTR rear derailleur should allow the 11x32 cassette to work with no other changes.

I have a triple on my touring bike. This Merlin is meant for randonneuring-type riding; ultra light long distance.

I love the simplicity of the dual crank. 50/34 (or 33) front with an 11/32 rear ought to be low enough. This setup sacrifices close gearing, but I'm not willing to sacrifice the 50/11 :D

Should a 52-42-30 front crank work with a 12/26 rear via a shimano 105rd with long cage? I took 2 links out of the 114 link 9sp chain because the cage didn't seem to be hanging right.

I use an ultegra gs (long cage) on my triple with a 11-32 just screw the b screw all the way in to make sure you have enough clearance and I'd probably avoid using the 50/32 and the 34/11 combos.

You got lucky. Most derailleur hangers won't let a Shimano road rear derailleur clear a 32T cog even with the B-screw all the way in.

Yes. It should work very well.

On a compact setup, why is it that you don't see a chain ring smaller than 34 paired with the 50?

Compact cranks almost universally have a 110 mm BCD (bolt circle diameter - the diameter of the circle on which the chainring bolts are set). This BCD will accept a 33T chainring as the absolute minimum and 34T is the most common size fitted by most makers.

If you want a smaller chainring, the next common bcd is 74 mm which will accept a minimum of a 24T chainring. This bcd is used for the "granny" chainring on most current road triple and older MTB triple cranks paired with a 130 mm or 110 bcd for the two larger rings.

Front derailleur capacity is another issue. most can only handle 16T though older models would be spec'd at 14T and still handle the gap of a compact. As stated, the BCD becomes an issue though.