I don't think IEPS has any conflict with the designation of 054X. Think about it: 055 is the benchmark destroyer line of PLAN and it's supposed to use IEPS in the future (maybe 055A?). I don't expect the future version with IEPS will be called anything other than 055X. The same thing here, the 054B designation doesn't prevent it from using IEPS. If it's called something else, say 057, that should be due to a new type of main engines, likely gas turbines.
If PLAN don't want IEPS at all, they wouldn't have mentioned it and Weiming Ma's team so often. Let's assume the several auxiliary ships with IEPS are actually some testbeds. Obviously, the next step, they should start testing it on real combatants soon. And I don't think it's a good idea to test it on some critical warships, such as 055A or 076. 054B is actually a perfect starting point. On the other hand, if 054B wouldn't incorporate IEPS at all, the changes over 054A should be somethings quite mature, such as new radars and missiles. In that case, they wouldn't need the new batch of 054A, at least not as many as 20 of them. Just add a few more till the first 054B gets launched. Then, they can simply focus on 054B after that.
They have not mentioned IEPS as of late, particularly in the last three years or so.
054X indicates an all diesel propulsion, as in diesels are the ones directly connected to the propeller via CODAD. Would you explain to me if there are any advantages of converting diesel power to electricity then back to mechanical power again? This means generators and electric motors between the diesel and the driving shaft. All diesel to electric should be called Diesel Electric transmission. That's what submarines do, and various trains. If its a gas engine to electric then should it be called Petrol Electric? If its gas turbine to electric, should it be called Turbo-Electric?
IEPS implies you need to have a combination of gas turbines and diesel engines, where the output is electrified and integrated to drive a common set of motors. It would make sense that a frigate with a combined diesel and gas turbine, using electric motors would be called IEPS. But an all diesel to electric setup should be called Diesel Electric and which is century old proven technology.
I also disagree with you about using a lower end warship as a starting point. Both the 056 and 054 series are as conservative as heck. Relative to their time periods, PLAN has been most technologically adventurous with high end platforms, starting with the Han class, the Type 052, then the Type 051B, the 052B, the 051C, the 052C and so on. It goes on to the 055 and the 003.
An IEPS with a combined gas turbine and diesel would have merited an all new designation, like 057 and beyond. Or succeed on a designation that has already used gas turbines before, such as 052X and 055X. I am more prone to think that the IEPS ship you are dreaming about is the 052X, perhaps the 052E.