more detailed spoilers below the cut but I can't stop giggling about that idea. I think, I have nothing to back this up, but I think Moffat wrote this entire two parter because of the Tom Baker clip that was featured in this episode.
the doctor accidentally stumbling upon Davros before he became the creator of the Daleks is an incredible idea. silly, but the show is often silly, and I really love the conclusion of it all that the doctor stole his chair and also let slip a lil regen juice to make the sewers revolt. did he know the sewers would revolt like Missy did? It's hard to say if he did that to Davros out of his eternal compassion or if he was playing 4D chess on Davros as he always does. My interpretation is he knew Colony Snake Man was disguised as the cables and did it on purpose cause he's a very clever lad, even though I think that's the more boring explanation. Missy in these episodes was so great too, they even explained the last time how she survived getting shot by a death laser and I genuinely thought she'd died again, until Clara got shot and y'know Clara has to be alive for later, but the play with trapping Clara inside a dalek shell was pretty devious. Missy was again testing the Doctor's willingness to kill, to do violence on his greatest enemies, yet he put down the gun and because of his oopsie and coming back to save Davros, the Daleks deep down know mercy, instead of only hatred. that.. that adds a ton of character to the daleks and I'm still super happy they figured out multi colored daleks at last. they just needed some finesse in the design and for them not to come out like the new model of volkswagen in all its different colors, these were just really, really good episodes.