phytoptera causes early death of the tree, it doesnt affect the quality of the timber, redwood and scotspine are fairly generic terms most redwood is probably larch. the term redwood, applies to pine that has a higher resin content. scots pine is generally a low grade of timber, fast grown so youll spot it by the space between the rings ive seen it with as few as 2 rings per inch, its crap, ok for rough temporary stuff but no good for proper work! the more rings per inch the better, that gives wood its lengthways strength, the darker ring is winter growth, the lighter ring is the growth put on when its not dormant.
a high resin content is good all round. its the woods natural preservative, it can make it harder to work tho, tools get clogged. scots pine is gnerally quite knotty and each knot is an area of weakness, timber is graded by its k.a.r, its knot/area ratio. joinery grade has alot less knots than the sort of stuff you would use for general carpentry.
ideally, what you are looking for is slow grown pine, but its getting harder to find and costsmuch more. this glut of larch atm is a good opportunity to get some reasonable timber at good prices.
im really suprised none of your suppliers have heard of tanalised timber, its everywhere! where abouts are you smudger? ill have a look. if you were near me i know of two sawmills that would easily supply what you need.
as you say any timber that is to be in contact with soil needs to be high pressure tanalised timber, ideally of a high resin content and ideally slow grown.