I have been farming DD Orbs from invaders for days. I can confirm DD invaders will appear after you have beaten a tower monster 3-4 times. I did the maths, every DD orb worth at least 1 gem or more and I get an invader around 15 mins. I just keep auto dueling 49/F with a meta Masked Heros deck (the worm deck is the easiest to bully), I have tried the Equip Spell deck and it works great too.
I get you point about wanting more DD Invaders, however I had all the DD Box rewards x3 at floor 35, I only want to finish the damn thing and these guys just keep spawning like that. Not sure if it is meant to work that way. I{d be happyto give them all to you