you might want to check out the relative lack of imperfects with πιπτειν. There is only 1 out of 90 verbs in NT, 10 out of 180 in Josephus.
The imperfects tend to be iterative, typically multiple deaths in battle.
Since I have not studied this verb in detail, my comments are tentative, but I am not convinced yet. Falling is something that often takes time. I don't think we have many texts about parachute jumping and all the things you can do while falling. Would this sound natural to you: ἄνθρωπος ἐπίπτει ἐκ δένδρου καὶ εἶπεν ἑαυτῷ τὸ πίπτειν οὐ δεινὸν ἀλλὰ τὸ παῦσαι πίπτειν? Do we have contexts that would call for a backgrounded process of falling where the imperfective aspect is avoided? If we do, then the conclusion that πίπτειν is an achievement is on more solid ground. If my example is not natural because πίπτειν is an achievement, then what verb will take its place in this kind of context? If πίπτειν is an achievement and there is no handy accomplishment synonym, it faces pressure to become an accomplishment in the right context. So I consider the achievement interpretation inherently unlikely, but I have not yet studied the examples.
Now Mark 14:35-36 is a nice puzzle.
is 'falling' and 'praying' a kind of rhetorical backgrounding? Or does it refer to an iterative falling and praying? (In other words, is the 'praying' iterative and assimilated to 'falling', or is 'falling' assimilated to an open-ended 'praying'?)
The whole passage is interesting in that it does not have a single storyline perfective between 32 and 38 until the second time of praying other than ἤρξατο in 33 (this is narrative!). My initial reaction is that we are dealing with a discourse peak that is marked with the imperfectives. If so, it may not be so much iterative or backgrounding. What do you think?