Is pressing on the legs for pleasure considered a forbidden form of masturbation that necessitates ghusl, and what are the characteristics of a woman's maniy to differentiate it from other discharges?

What you are engaging in is a form of forbidden masturbation, as it is considered an act of aggression mentioned in His Almighty's saying: "And those who guard their private parts (5) Except from their wives or those their right hands possess, for indeed, they are not to be blamed (6) But whoever seeks beyond that, then those are the transgressors." You are obligated to sincerely repent and resolve not to return to this sin. If this results in the emission of semen, then ritual bathing (ghusl) is obligatory. If it is pre-seminal fluid (madhiy), it nullifies ablution (wudu') and is impure, requiring washing whatever it touches.