Severus did not bother to continue with that train of thought. Whether Theo had truly taken sufficient steps or not was hardly his concern. He had enough to worry about with his own son as well as Draco's family. He had no time or inclination to take on anyone else.
He grimaced momentarily as he considered the pros and cons of informing Rodolphus of that particular piece of news. He would not be pleased, not after receiving confirmation of Draco's disloyalty. Umbridge probably suited Rodolphus better than Draco ever would and there was a risk that he would warn Umbridge before Draco could consolidate things sufficiently.
"Do it," he said after several moment's thought. "If he refuses a meeting as I suspect he will no matter which path he is taking, push for one anyway. If you can do so without him knowing, try and see if he is using glamours." Of course the difficulty would be in knowing whether the glamours were being used to disguise an illness or mimic one but it would be interesting either way. If he was truly ill, the glamours would likely not have the same power than if he was simply creating a façade.
He arched an eyebrow at Theo's question. "Rodolphus, Bellatrix, Rabastan, myself... likely others. Most amongst the Death Eaters do not go around advertising their ability in the Dark Arts so there are undoubtedly others that I do not know of. Look to those with ambition, someone who wants to climb the ranks and impress Rodolphus enough that are willing to find and learn such things secretly."