It would make more sense if it was Jennifer, but I figure it is for drama reasons also. I could understand if Jennifer and Hope had a scene together, and Jennifer had to decline because of the mess with her resigning from the Horton Center. Since Hope is a cop, having a family member who has been rumored to be involved in drugs standing up for her could be seen as problematic professionally. Then the choice of Kayla would make more sense. I figure it has to do with Bo being missing or Steve returning etc.
The only reason I'd be concerned if they made either Mariah or Kevin asexual is due to the abuse they went through, as that's how they would potentially excuse or explain it to people who don't understand. That would be totally awful.
The character of Sonny and Maurice Benard has been with GH since 1993. Regardless of Sonny's divisiveness, he falls into the "vet" category on that show, due to the longevity and the fact there aren't many people that pre-date him that are still on canvas. Yes there are actors who are "used" on GH, that started before him, but most have huge gaps in their time of service or are barely featured (like Mac who appeared two years earlier and Felicia who was off canvas for many years). The character of Carly is celebrating 20 in 2016, so while Laura Wright has only been on GH for 10 years this autumn, she isn't exactly a newbie either.
To me casting or even sets aren't that big deal when it comes to spoilers. People aren't periscoping actual scenes that are filmed outside and adding subtitles so everyone knows the dialogue or something of that nature.