Being someone who sings it everyday, I can confirm to you that this "Weihrauchpsalm Zur abendlichen Weihrauchdarbringung " IS a gregorian melody. The melody resembles the average antiphon that would come before a canticle or psalm.
In particular the melody reminds me vaguely of the Antiphon on Magnificat at II Vespers for Nativity of Our Lord "Hodie Christus Natus est" (Today the Christ is born).
"Psalm 28 Aus dem Morgengottesdienst (Orthros) " - I can not tell which form this (I'm going to guess valaam?), yet it is very similar to the tone that the Gospel is sung to in Gregorian chant, but not psalms or canticles that I know of.
The Morgenprokimenon and Polyeleon are more valaam sounding, however it appears there is whether conciously or accidently an attempt to blend some of the different styles together. Sometimes when one person sings another kind of chant they accidently carry that influence into another form (that always creates some controversy with purists).
I suppose their blending of different styles or influences is successful so far. I am impressed.
I have never ever witnessed anyone doing this before in the Orthodox Church, a few western rite orthodox churches have probably tried to do this but usually not very successfully, past attempts tended to sound much more awkward than these monks results.
As Marcel Peres first noted in 1993 (Gradual of Eleanor of Brittany CD), Some of the late medieval faux bourdons and 2 or 3 part polyphony and harmony of the latin church sound very similar to sound of the harmony in the slavic churches. That is an area I am trying to focus more on and accumulate settings of, rather than only the monodic plainchant versions. (theres also techniques to learn to harmonize by yourself without notation)
Example: Rex Caeli. Organum paralelo modificado