Does seeing a woman in a dream with a man in bed without intercourse, then being unable to distinguish the fluid due to the use of an absorbent diaper, and considering the fluid to be madhy, necessitate ghusl or not?

Ghusl (definition) is not necessitated by mere seeing. Rather, the emission of semen, which obligates ghusl, must be ascertained. If you are uncertain about the fluid discharged, you may choose to consider it madhy (pre-seminal fluid), and there is no sin in doing so.