questions on replaceing roof deck

1 story addition on a 2 story house. roof i am working on is about 2' below 2nd story window. very little pitch.
8' wide by 40 ' long.
roof leaks. has been for years. tried everything, i think it was just poorly designed. water just cant get off it fast enough. going with all new plywood and a paint like product by AMES.
my questions, they used 6" wide pine boards when they built it ,for a roof deck. framed with 2x6's. this was built around 1965. the 2x6's are just shy of 16" OC. can i just put a 2x6 against the existing 2x6? where the tape measure is in the photo. i would nail the new 2x6 to the existing 2x6 to make it wider so the plywood would have an edge to rest on.
also, on large house roofs with normal pitch, i also see the plywood being put on length wise. meaning each row advances to the peak 4' at a time. the first row is full sheets, second row starts with a half sheet to stagger the joints. since my roof is 8' wide, from edge to wall of house, can i just use full sheets width wise? or should i use 2 rows length wise, staggering the second row? just wondering if it mattered on a smaller roof.
and, you will notice in the photo only part of the roof is stripped. i realize most people would just strip everything first, but i have learned this house holds a lot of surprises. i think it will was built by drunken farmers. i demolish a little at a time. if i find a terrible surprise, i could stop and leave the roof on most of it and only have to worry about tarping a small area.
thanks.

 

You can add a 2x6 as a nailer wherever it's needed. I don't see a problem with using full sheets of plywood but wait for other responses.