Hi,
Neither the letters nor the memo (was) on your desk.
Neither/ either/ any/ someone etc.....always takes on the singular verb form
I feel really (bad) about what you said to me.
The sensing verbs are the only exceptions that can use adjectives
Ex: It feels good , I fine fine
She looks terrific, He looks weird
The wine tastes too try.
The accident was (due to the result of ) the driver's negligence. personally, I think due/ because of are understood. But I gave you another option to consider.