with 5/16 engineered hardwood floor, how much of a gap between wood and wall is necesary if it is important at all.

I'd leave a 1/4", just to be safe.

Half that would be plenty. An attached wood floor is destroyed long before it grows enough to close a gap equal to its own thickness.(in most cases)

Last one we did was by Mohawk and the install instruction said " no expansion space" I couldn't believe it.

I can. Check this out: I've done many renovations in older homes. Some had walls that were plastered (not sheetrock, real plaster) AFTER the baseboards were installed, meaning the walls were plastered to the baseboards. Then the wood flooring was cut net and tight to the baseboards and the wood was solid oak, plain sawn. Go figure!

Mannington has had no expansion space needed, in their instructions, for 5 years now. And now I see they say no acclimation is needed.

Mohawk wood says no expansion as does mannington. During season changes wood expands and contracts, somethings going to give, right?

If the floor fails do to the wood in the walls expanding whos it coming back on?

Mohawk used to say no expansion needed on laminate. The last job I did with that junk said leave 1/2" expansion and over certain footages leave an additional 1/6" per quoted footage.

Mannington requires 5/16" expansion on there bamboo floors.