Hope this doesn't count as a Necro, I'm new here but I really enjoyed reading that, the color coding on the diagrams makes your breakdown very clear.
Two things I thought of while reading it/during my own playthrough:
1. Those crossbow guys often act as a signal to the end of a section. Looking back through the series I can think of quite a few times a CB enemy is the last one before a bonfire. I would consider this for three reasons - the aforementioned bonfire sign, the reward of an easy enemy to finish with, and a punishment for unaware players. How many times must I have died relaxing after a fight just to hear that 'twang' sound. If you haven't seen the crossbow, you haven't finished the area!
2. I feel gundyr being still is also a clever double bluff, and a doubling down on the playing dead you mentioned. On first sight the player (due to the previous section teaching them) will assume he is simply faking his death, most likely very tentatively approaching him thinking "you won't get me this time". But once you reach him nothing happens! The player then relaxes, "maybe this guy really is a statue,or clearly dead with this polearm through him". So they hit the prompt, maybe expecting a cutscene and what do they get? He really was alive all along! This has the great reward of catching unsuspecting/new/trusting players out, or giving suspicious/seasoned that 'aha! I knew it!' moment all while reinforcing what you said on never trusting an enemy in ds3 unless you killed them yourself.