This was absolutely brutal. There is almost no way to proceed but to cold solve everything, or at least until you can get the message about grid entries. Even then there is little help from crossing entries, as will become clear.
1a = TAR TAR, C is extra.
I eventually got the message by writing all the possibilities for extra letters out and staring at them until words appeared. I am loth to declare a puzzle unfair, but this one goes right up to the line.