To reduce risk of battery arcing , as soon as the old battery connector was detached, I wrapped it with insulation from connection all the way to the battery body. Did the same with the new battery to eliminate concerns about shorts while checking fit position to assure the connector would match up with the receiver. Only removed the insulation after all other connections had been restored, so plugging in the battery was the last step before closing the case.
Thanks to Heather Pastushok (and Behrang Mohajer at “iPhone battery not charging . . .) for this important detail. My iFixit battery came with a thin layer of black foam insulation on the top surface of the battery connector, but even gentle pressure from a blunt plastic tool (pushing the connector in place) scraped an exposed spot. A carefully trimmed piece of black electrician's tape provided peace of mind. The iPhone 6s is good as new with the battery now at 100%!