I mentioned also that coordinates and times could be offset by a predetermined amount, and be different for each U-Boot.
As for times, my understanding that they would, at least sometimes, specify them in the form of so many hours after a specific trigger message was received. This seems to require the U-boat to be surfaced (or maybe just at periscope depth?) from the start of the window for this message until it was received.
None of this addresses the training issue from earlier. The thing is, if you are thinking of using a newspaper-based scheme only for U-boats, you might as well go the extra distance to implement a true OTP system, which seems feasible at that scale and which would turn the use of Enigma in series into an academic exercise.
BTW, I've used dice rolls to generate passwords, but the dice would always fall off my desk and roll into an inaccessible corner, and I soon tired of that. It also only worked for 6 digits! So I switched to a real RNG.
As for key interdiction, it would have to be at massive scale, as having a handful of random keys does not enable industrial-scale decryption, which is what the allies needed (and had, some of the time) for signals intelligence to be useful against U-boats. In contrast, the leaking of one date would compromise the entirety of the communications conducted under it (i.e., at least those for one U-boat for the duration of the patrol), especially given that German newspapers were readily available in neutral and occupied countries. OTPs are, in fact, way more resistant to espionage than your newspaper scheme.
The failure of the location-shifting scheme (as mentioned in my previous post) to stop the allies interdicting rendezvous, despite its periodic changes, also suggests that one date per patrol is not going to be good enough.
We have reached the point where the alleged superiority of your newspaper scheme hangs on the tendency of dice to roll off your desk!