"Terrific" is closer than "cool" to the modern meaning of "toll".
The old meaning "toll" = "mad" is outdated and forgotten - it just still exists in a few combinations like "Tollwut" = "rabies"
English "cool" in German is "cool" (often pronounced with a slightly different "oo" -more straightforward, less of a diphtong - and the "l" a little bit more to the front of the mouth.)
Why is it that sometimes ist is used for are instead of sind or seid?
ist is third person singular -- he, she, it.
Do you have an example sentence you are thinking of where "sometimes ist is used for are"?
Besides words where German uses a singular word but English uses a plural word, e.g. die Hose ist blau = the pants are blue / das Gemüse ist teuer = the vegetables are expensive?