Were there German U-boats in Eastern Mediterranean? And where would they be based? They should also stay well out of Turkish territorial waters.
The pile up is ok, as is the mining of the straits, but don't push it too much.
Yes. It is the UB and UC classes that were shippable by rail. My understanding is they were put in the water in the Adriatic then later based out of Ottomans lands. If you look at the distances, it is not as bad as getting to the Western side of the UK. And interesting fact from OTL is that the Germans used USW rules for the entire war once ships went North of Crete. No big issues ever arose from this USW zone. The territorial limit is 3 miles or so, so this is not a big issue. The waters are very congested for at least 100 miles out.
Now to what this TL looks like assuming both sides (Italy and Ottomans) try to stay neutral.
1) Most likely, the Ottoman allow no ships into the straights for durationof war. This is a key point. IOTL, the Ottomans banned all shipping in straights while still neutral and deciding if to enter the war.
2) But ok for discussion purposes, I grant Ottomans will allow ships to pass through straights after inspection. All neutrals follow rules of maritime law back then. I guess Greece stays neutral. Not stated in question, but if Greece is Allied, Germans can just operate freely in Greek waters. So what happens.
A) Basing, probably most U-boats go back to A-H. The blockade of the Adriatic was not effective versus U-boats IOTL. If Italian forces don't help and UK stays carefully out of Italian waters, the barrage might as well not exists. If we take the rules back then, seems like you could stop at neutral port for 48 hours per ship per country per 60 days. U-boats can sneak out of neutral ports easy enough. Food, fuel and others don't take that long. I have Greece to use, Bulgaria, and Ottomans. Likely U-boats can operate indefinitely without going back to Adriatic. In all likelihood, small smugglers will supply from neutral ships. We are basically talking a little fuel, food, and perhaps some ammo.
B) German spies will be radioing out which ships exit straights when. German navy will have list of target ships. Sure British spies will also be there, but they can't have as big an impact.
C) Tight, narrow waters with know targets. If we assume observing neutrality of neutrals, UK escorts have to do it too. Either you keep the convoy well outside of Greek/Ottomans water the entire time or ships are unescorted. Also, you can't arm the merchants or now AMC.
D) Best hunting will be where these merchants pile up waiting to enter straights. Now since these subs are rail mobile, I can also do in Black Sea and only face Russian Navy. Same idea, different Sea.
Really, very little change here from OTL where Germans hunted quite sucessfully in this area. Actually in many ways, much easier for Germans since easier to hunt in Aegean/Black Sea than west of UK.