I received a part 3 summons, signed by a JP, with no date on it. I have been to court several times, but not to trial yet. Section 24 (2) of the POA states that 'A justice shall not sign a summons or warrant in blank'. The prosecutor refuses to withdraw the charges, says they can amend it. Is the summons considered a continuing document as well as issuing? Should I go the mandamus route in Superior court, just not show up for trial or deal with the JP at trial? Any thoughts?

If you appeared in court after receiving the summons, then it has served it's purpose. A summons is just to get you in the door at court. It's no longer relevant in my opinion.

You've since been to court many times. I would presume you were instructed on court record by the JP when to return to court next. Failing to appear would mean they could proceed with a trial in your absence or issue a warrant depending on the seriousness of the charges.

It states "and to appear thereafter as required by the court to be dealt with according to law", does this not give it continuing force?

Since it was illegal for the JP to sign in the first place, is that in and of itself not fatal, no matter what stage the proceeding is at?