I think #484631|Ore wa Miyamura sensei | I'm Miyamura s…|4,047 should also classify, because the person possession another body is the only one active / with a "soul"/mind.
Now, if its possession swap, as in the minds swap, then its no longer solo action.
Agreed, though I'd like to offer some specifics.
1) in order to count as solo action, only one character can be involved in any lewd activity. This can include, for example; masturbation, scat, urination, exhibitionism, voyeurism, etc, but cannot feature another character being actively involved.
2) Other characters may be part of the story, but cannot be part of any sexual activity. This includes things like a friend who interrupts the protagonist while they're masturbating, a colleague at work who says goodbye at the start, or a farmer who finds the protagonist masturbating with a corn cob and chases them away.
3) Doujins that feature solo activity as part of a bigger story that features other characters are disqualified.
4) situations like phone sex where one character calls somebody to get off are disqualified, as the person on the other end is a participant, but something like webcam streaming counts, so long as the focus is strictly on either the streamer or one viewer.