idk once you get into the habit of it douching is just kind of normal to do before you get out it doesn't necessarily imply anything nefarious. I douche before going out to parties and my gf when staying at home sometimes has gotten a little jealous auto-assuming that it means i'm planning to bottom that night. I always just tell her it's because I'm planning to bottom for her when I get home later.
Yeah it is literally impossible to get all the cum out though. It is just so resistant to water-solubility having such a high concentration of gel-forming proteins. I think the main enzymes supposedly break down after like a half hour or so but the jizz continues to leak out all day long for at least like 14 hours. I think it gets into pockets in your ass and mixes with your own ass fluids.