Is thinking about sexual matters that leads to body tremors, without the emission of anything, obligating ghusl? And does the emission of a clear, viscous fluid, suspected to be madhiy or maniy, necessitate the re-performance of past prayers?

The mere feeling of pleasure does not necessitate ghusl (definition) unless that which necessitates it, namely semen (mani), is discharged. The fluid described as madhy (pre-seminal fluid) necessitates wudu, not ghusl. As for someone who is unsure whether the discharge is mani or madhy, they may choose between the two according to the opinion of Shafi'i jurists.