fw 190a5 flight model

[Glider]

Originally Posted by Gaston

-On April 29th the regiment completed 28 sorties to escort bombers and ground attack aircrafts and 23 to protect ground troops, with four air battles occuring. The first few days were marked by failures due to the use of "outdated" (my use of quotation marks) horizontal combat tactics (My note: horizontal combat was never considered outdated in all of WWII, except for the Allies in the Pacific: It covers about 95%+ of all Western air battle in 1944) while the Spitfire was particularly well-adapted to fighting in the vertical plane.

Wrong
Horizontal combat was not the norm in the real world and to say it covered 95% of combats is a farce. Also it was in many ways outdated. Turning is mainly a defensive tactic and fighters are designed to attack, most combats were in and out and a high proportion of pilots who were shot down never knew what hit them. Height in combat is a vital advantage and the Spit was good in both climb and turn.
You can of course support the 95% comment?

Translation: "At low and medium altitude, the Mark VB was outperformed by German and Soviet fighters of its time. To try to improve its maneuverability and its speed (?!?: My note: They couldn't have expected much speed increase from that now could they? Obviously this was more about maneuvering), the Soviets lightened it by removing the four machineguns and their ammunition. This variant was evaluated by the VVS test center during the Summer of 1943. Apparently it was not a success, as there was no instruction to standardize the modification"

Nothing unexpected here the Mk V was outclassed by the Fw190 and the 109G

If the turn rate was really satisfactory to the Soviets compared to their own types, why would they change tactics to the vertical for this type alone? And why did they try to lighten it, at no improvement in drag or speed, if not obviously to improve its maneuverability? If the Spitfire really turned with around 17-18 sec turn times (TsAGI), which is every bit as good as the best of their fighters, why did they consider it unsuitable for their ususal turning tactics?

The Russians didn't consider the Spit unsuitable for turning combat as proved by your own words which I will quote later

Except against slow-turning types like the P-51 or the Me-109G, turning tactics with the Spitfire were simply not very competitive, this worsening with the Mk IX, which is why the Mk IX is always used in dive and zoom tactics (followed by the occasional harsh high G high speed unsustained turn, its performance for which was on the other hand quite good), and this almost without exception: The vertical was what it excelled at...

As mentioned before this is wrong the SPit was good at the turn and the climb.

Your Russian quote
The regiment’s pilots considered the conduct of battles in the horizontal plane to be the optimum method of contesting with German fighters. Despite the fact that, as already noted above, because of its lightness the Spitfire was a quick climber, the pilots of 57th GIAP recommended engaging the Messers and Fokkers in turning battles. Ivanov emphasizes that it was necessary to draw the enemy into a right turn, “because the Messerschmitt’s propeller rotated to the left, and the airplane executed right turns with greater difficulty than left turns.” For this reason, the regiment’s pilots mastered the execution of deep right turns in the Spitfire. In Ivanov’s opinion, this training was no accident, and many enemy fighters were destroyed using this particular method.

Which you translated as
It does seem that drawing into a right turn seemed to be emphasized by the pilots here but that horizontal fighting was recommend method by the pilots of this Russian Guards unit.

I am sure you meant to say
It does seem that drawing into a right turn seemed to be emphasized by the pilots here and horizontal fighting was recommend method by the pilots of this Russian Guards unit.
Because Horizontal is left and right
So to sum up the Russians also recognised that the Spit was good in a turn and shot down many enemy aircraft using that tactic.