So you think they'd convert a known torpedo/ammunition transport ship into a covert torpedo platform, with a torpedo mounting that would be blatantly obvious to a NATO submarine doing a hull search, as opposed to one even remotely hidden, and that it would have sonar which is logical only if its going to covertly attack submarines, and then to round things off they assign it a hull number which specifically designates it as an experimental vessel rather then retaining a number of another torpedo/ammo transport? Does that really make sense to you? Feel free to point out any errors of my thinking, but, it seems pretty convoluted.
Now if you think the Soviets were really worried about the safety or cost of placing two underwater torpedo tubes on a small transport for tests, well, have you noticed the Kirov class, or Chernobyl, or all the subs they built that sank? The Soviet Union was one of the most unsafe and excessive places you ever could dream up, and the Red Fleet had hoards of unusual auxiliaries already like dedicated liquid missile fuel transport ships.
As for broaching, if you think this tube will come out of the water, how do you think it would remain a covert platform if it was steaming on the open ocean? If you wanted covert torpedoes for an ASW role that just doesn't make a lot of sense compared to say, a submerged broadside tube in a freighter with an retracting hull sonar. Such a setup would actually be pretty hard to detect even with a hull search. This ship meanwhile is as blatant as you could be. I wouldn’t even have a normal bow wave because of the knuckle effect of the big blunt tube door. For covert anti surface attack above water tubes would be much cheaper, and you have no need of sonar in any case.
Also you are incorrect on depth for other nation’s trials platforms. I just looked up the US Cape Flattery class trial ships, and they had 21in tubes and only 10.5ft draft, the stock Project 1824 Muna draws 8.5ft. Cape Flattery was designed from scratch for the torpedo trials role, so had more depth been any real importance they would have had it. The USN bought no less then four of them too. Even if this ship draws a little less water then a stock Muna we are talking about a difference of perhaps 1 meter of draft. I have a very hard time buying that this could be relevant. Conduct trials in good weather and it will be fine. You can choose when to fire torpedoes in trials, a useful covert operations platform could not be dependent on calm weather.
Also notice how it has a crane, and was designed to transport ammunition. That’s a nice feature for recovering test torpedoes. Its very logical to turn a small ammunition ship into a trials hull, I can't see any specific logic that would support using it for covert operations, and many reasons I've listed why it doesn't make sense. Certainly the Soviets had many secret plans, but this doesn't look like one of them to me. My guess is it has sonar so it can do trials with wire guided torpedoes.