Partial answers

"How many years did the articulateds run on the SV?"
-SVRy purchased them in 1939, or maybe early 1940 and used them until the end of mainline operations in 1947.
"How long did they run with their sidetanks and tenders? Why did the SV leave the sidetanks on after adding the tenders? Was it an experiment to see how the tractive effort would be effected?"
-I don't think this is known for sure, but probably not for long. Most likely the side tanks were left on initially for ballast/tractive effort. Or just in case the conversion didn't work out and had to be converted back. Or both.
"By any chance were these behemoths ever used in passenger or mixed service?"
-Technically yes. After the passenger runs were ended (1937) SV continued to accomodate paying passengers in the caboose on regular freight runs. Far as I know the mallets never pulled varnish, however.
"Final question. How long were the 2-6-6-2s with the tenders added?"
-Not sure, but they were 45'-8" long in their tank configuration. Maybe Jerry or somebody could chime with the tender dimensions.
Hope this helps, others can probably provide more accurate information.
J