Are we still talking about SMPlayer? 
SVP rev.191
note the socket path in use!
1. old SMPlayer (before ver.19.10.0.9290 from 2020-04-06) + old mpv (before 2020-03-26)
SVP log:
13:36:16.860 []: VideoPlayer: mpv 0.32.0 /via 'mpvpipe'
only one instance will work
2. new SMPlayer + old mpv -> doesn't work at all
3. new SMPlayer + new mpv
13:38:46.648 []: VideoPlayer: mpv 0.32.0-322-g5824ac7d36 /via 'C:/Users/chain/AppData/Local/Temp/smplayer-mpv-1adc'
socket name is specific to the SMPlayer instance
4. mpv with fixed "input-ipc-server"
13:36:16.860 []: VideoPlayer: mpv 0.32.0-322-g5824ac7d36 /via 'mpvpipe'
only one instance will work
5. mpv with mpvSockets script
13:44:35.210 []: VideoPlayer: mpv 0.32.0-322-g5824ac7d36 /via 'mpvSockets/20748'
socket name is specific to the mpv instance