Would you electively treat the neck for a completely resected parotid carcinoma ex pleomorphic adenoma with negative lymph node sampling?

Yes, unless there’s a reason not to. For that matter, if it’s a high grade carcinoma and you know you’re going to add postop RT, don’t do the neck dissection and irradiate the neck. It is as effective.

For positive margin disease with aggressive SCC tumor (PNI, LVSI) involving up to the stylomastoid foramen, and neck nodes negative (except for an involved 'neurovascular bundle' at lvl 3), what doses would you use?

High grade, yes. Low grade, no.