It appears that you pretty much have your answers correct.
True the brothers Kelber were a huge tube source for the brothers Merkel and of course others. The brothers Merkel were very able and could adapt to very narrow time constraints. If I recall correctly they could produce a custom drilling in about 5 weeks, if need be, which is pretty remarkable to say the least. But they did have their sourcing lines as well as production lines firmly established. The DR, which is some sort of Kerner-Anson or A&D(maybe a model 142???), has the intertwined "WK" in which the "K" is encirced by a "W" which appears somewhat as a heart on the Krupp cast steel tubes. Some subscribe to the thought that the mark denotes that Wilhelm Kelber fitted the tubes to the action/profiled and/or finished the tubes. But why would there be a separate mark on each individual tube? The reason is that it is the forge mark of Wilhelm Kelber while the script initials "W.K." near the forend lug note that he fitted the tubes to the action was well as finished/polished/rifled/profiled the tubes. The proof date for the DR is February 1931 thru the Suhl proofhouse. The triggerbow guard is horn and the father of Ernst August Merkel, of the firm E.A. Merkel, Ernst Ferdinand Merkel was an engraver.
Merkel's A&D drilling had a model number of 141 & 143 but I can't remember the Blitz model right off. I don't think the tubes of your Blitz drilling are nickel plated. The frame either has a coin finish or is an alloy frame. The acquisition of Gottfried Funk, or some portion thereof, in 1925 might explain the alloy frame. The encircled 16 would be the gauge and notes 65mm chambers, was well at one time also noted black powder but the scattergun tubes have the Suhl Nitro stamp. There looks to be another stamp, or scratch, just below the "72" near the forward barrel lug. Can you verify or snap a better image? I don't see a date stamp but would date it post 1912 and pre-1923.
The "D.R.G.M." on the cartridge trap is for some small innovation thereof.
Kind Regards,
Raimey
rse