Ok, I got the replacement set of Shimano Ultegra FC-R603 rear cranks today and installed them. A first time installing this style timing-side crankarm which is similar to a XTR M970. After installing it I found out that to remove the arm needs a rather expensive ($38) TL-FC35 tool
I also learned that there is no preload adjustment on these cranks. I guess that is similar to FSA cranks that only use spacers to take up any slack between the bottom brackets and crank arms? I ended up with just the washer/spacer on the timing crank side, and added both a .6mm + .1mm spacer on the drive side. This was the amount of spacers determined by the tester tool Shimano provides. The end result seems to align the front and rear timing rings to roughly within 1mm (quickly measured from the seat tubes outward). I'm stuck with the rear crank setup for a few days until the TL-FC35 tool order comes in.
One other slight gripe about these cranks is the open hex hole end in the timing crank bolt. That provides a nice entry point for grime & water access to the inside the axle. For peace of mind, I'll have to devise a rubber plug and/or silicone blob to seal that (BTW, this is the same type of cruddy design that the XTR M970 has... with the open-ended crank bolt).
Good news about Q-Factor. I measured (rough) 150mm on the front/captain, and 159mm on the rear/stoker. My wife is really happy to hear that about her cockpit, as it's only 4mm wider than her road single w/triple. Our previous Santana had her at a tripod stance of 172mm which is really tough on a 5'2" stoker. Heel clearance is now identical to her single at 2.5cm, unlike the Santana which had 1cm less. So things are looking really good for biomech leg setup.