"Do I crack them all at once or one at a time?"
Since you're looking for fuel flow on a non Starter it doesn't matter.
Put some rags around/underneath the capnuts in case you are Lucky.
"Just to be clear, I am soaking the glow plugs in situ? Also, is there a way to get at those while the intake manifold is in place? If so, how?"
1. Yes,You're just Trying to head off what MAY be a Problem.If you encounter ANY resistance when Loosening STOP and use
1/16 turns back and forth on the GP to try and Coax it free.
2.I don't have the W201 manual.
(I've no idea what this is 'turned up on a Google)
http://auto.manualsonline.com/manuals/mfg/mercedesbenz/mercedesbenz_automobile_product_list.html