I do not see a Q28 on the schematic, and in fact there is a note on page 1 that the highest number used was Q27. SO I don't see how it is missing. If the board has Q28 printed on it, perhaps another model uses that board too, and Q28 only exists on that model.
TP21 obviously was meant to be negative, just a misprint.
I believe schematics until I have a reason not to. For example, if there is an error drawn in the reverb circuit, and I am not working on the reverb circuit, then I no longer care about that drawing error.