Re: Joel, Kiko, Miko (girls)
I've not made any studies on the subject but it does seem that *most* boy names stolen for girls contain softer sounds and components that are already familiar in girl names. Elliott is an example of this. As much as I see Elliott as completely masculine and hate to see it used on girls, logically, it does contain the "ellie" sound, and an ending similar to the "ette" component commonly used in girl names.
I'm one who does not enjoy boy names on girls at all but, regarding the specific examples you list here, Devon does work on a girl for me. This is influenced by the softer sounds the name contains and by the fact that I strongly associate it with a beautiful place and thus with feminine pronouns since, by tradition (for English-speakers, anyway), cities/countries/vessels, etc. are referred to as "she"/"her".
Ryan and Rowan are completely masculine for me but, logically, I can understand how some people might perceive them as having feminine qualities because they contain softer sounds and similarities to already familiar girl names, such as Diane and Rowena.
For me, Joel cannot really be placed in the same "logical" category. Aside from the potential nickname "Jo", there's nothing here that feels recognizable as similar to already familiar girl names. Sound and feel-wise, for me, Joel is overtly more masculine than either Ryan or Rowan, so it makes sense to me that people would perceive it as a stranger choice for a girl.