You will get normal benefits on KL (seat selection of any Y seat including Economy Comfort, SkyPriority, lounge access in MAD and AMS).
For the trip back, assuming you booked a KL-marketed flight, you will get standard and preferred seat selection on DL and SkyPriority. You will only be eligible for Comfort+ upgrades day of departure.
You will not receive Sky Club lounge access in BOS since you are flying Economy, but you can use the Air France or Virgin lounge.
On KL operated, only the member with status gets preferred seats and seat assignments for free. on DL, should be able to get seats in advance (incl. preferred). I believe bags are the same (additional allowance only for the status holder), but YMMV.