Yes, Thomas was often written as "Thos".
It seems unlikely, but in handwritten wills "Wm" [William] can look very like "Thos".
"Jno" was often written for John, especially in early land records, but I've never seen it used as an abbreviation for Jonathon which seldom appears pre-1800 in northern IOM registers. Jonathon/Johnathon appears once only (as a second name) in the Maughold bapt register pre-1849.

Sue