I have used Paraguard with Otos at full strength as proactive treatment with no ill effects.
Salt and Otos are not a good mix, however, it would add stress to a soft-water fish whose immune system is already compromised by the parasite. Now, if it was a Platy or Molly, sure- added salt would be very beneficial. But, with fish endemic to low/ to almost no salinity waters, the only time I would say salt is appropriate is in the case of nitrite toxicity ( to counteract "Brown Blood disease".).
There is a heat- resistant Ich variant that is becoming more and more frequent in ornamental fish. because of this, I use medications. There are no Ich variants resistant to malachite/Victoria green, the active ingredient in most proprietary blend Ich medications. If the med was ineffective it was user error or a misdiagnosis in the first place.