My paste goes reliably 4 months. I leave it to set 15-30 minutes on the surface.
As for the patches; Meg's had a problem with the consistency of early NXT & #21 emulsions. If the products are from the older batches it is possible that you'll get smeary, slightly oily look, with stubborn, dark-looking patches.
That's a kind of overapplication, because the active ingredients can not level well, and are unable to establish a continuous, even film on the surface. The dark patches are indeed movable with a towel. You should avoid over-deposition of actives - so simply try to use less when you have older formulae of NXT & #21.
Earlier #21s tend to oiling out from the emulsion, releasing "water" droplets during application.
Megs will replace the products if the issue is too disturbing. New batches don't have this problem, and of course, because of its different composition, the NXT paste was never affected.