I am willing to say with extreme confidence that Stevie is not playing any of the synth parts on Everywhere. She was barely there to record a coherent vocal in 1987 let alone lay down synth parts on a song she did not write.
If you look at the personnel credits on Wiki for Tango in the Night, Say You Will, EP and Buckingham McVie they are increasingly Buckingham studio craft pieces that went beyond producing and arranging to actually playing and programming lots of the parts himself. This may have been a point of consternation for the band during the production of these albums, but maybe not? Although I believe he is the principal architect of the band's gorgeous ersatz symphonic sound, sometimes the architect has to know that the mason lays better brick than he himself would.
This is a great article on Tango and the building of that album.
https://pitchfork.com/reviews/albums...eluxe-edition/
In Tango's case I think the physical and mental condition of the other band members forced his hand with many of these choices.