I wonder if their chibi forms are basically a weakened version of vampires' abilities to turn into bats. Like maybe their bloodline is thinning which is why they can also survive in the sun. Also using chibi forms to conserve energy probably
Guessing they will reveal at the end that she sucked on him so much through the story, that he becomes a vampire, avoiding the whole issue of her seeing him die of old age.
Regarding page 5, it looks to me that is a rather formal, planned painting. So it could still very well be Ootori's idea to capture the more spontaneous moments on a canvas!
Edit: confirmed literally 5 pages later lmao
Also, what is this supposed to say? I can't figure out "This was the given with us".