I spray my plants every week in veg.with either neem,Protekt & dust with DT this seems to keep them away for me without spraying in flower.Marco,Thumper or Getaway would have the best preventative program for your area.
looks dry to me also.In hot weather some plants use a lot more water than others.I have three plants in a greenhouse and water them all at separate times using a moisture meter to determine when.
Your plants should stretch before the rain hits and give you a good idea on how high to make the cover.It's always a crap shoot when you grow outside so many variables
A 3/16in. wide piece of duct would probably prevent that where it makes contact and not choke the stem. My cages are galvanized some of the blackness could be from that it tends to leave marks on things it comes in contact with.
I went out and looked closely at mine,the winds blows here all the time.I have that on two of my plants in tomato cages. some of it is black and the older abrasions are brown and look like healed super cropped knuckles in color.