Squid and Hedgehog in the late-war escort fleet

Was the Squid anti-submarine mortar deployed early enough to see Second World War service?

Yes. About seventy ships carried Squid by the war's end, although Hedgehog did more of the wartime work.

Squid was an operational wartime weapon rather than a purely postwar system. About seventy ships carried it by the end of World War II. That deployment count still overstates its share of the campaign, because Hedgehog did more of the wartime work.