Is 5/8" plywood over 1/2" board subfloor adequate for tile?
Your plan is reasonable, but I'd make a few suggestions...


Most tile and mortar manufacturers want 1-1/4" minimum subfloor. I'm not sure if the Ditra helps with that, but you probably want to go with 3/4" plywood. 
1-1/2" screws are likely to strip in that aged wood. I'd use 2" or 2-1/2". They need to sink flush or below.
You don't need expensive coated deck screws. Plain gold construction screws will do. Don't use black oxide drywall screws. They're too brittle.
It's not so important that you screw every 6 inches as much as it's important that every board is down securely. Screw about 3/4" in from each edge of each board. 
Consider a small-bead zigzag of construction adhesive under the plywood, perpendicular to the boards below. This will help lock in any movement (which makes noise). A screw pattern of about 12" on each joist should be plenty for the plywood, then.