It's the same issue in Double Exposure. It would be difficult for Kepple to walk into the theater to murder White without being seen.

Yes! Unless he had a key to a back entrance or snuck in with a group or something, he would have to buy a ticket in order to enter the theater. The Magnolia was showing "High Plains Drifter" - a very popular movie that year. It's highly likely someone would have seen him entering the theater and it's likely he would have been seen sneaking into the projector's room.

Robert Culp characters on the show have a few things in common.
Even though one of them does it accidentally, two of them are blackmailers who kill their blackmail victims - "killing the golden goose" (as Jerry Parks would say).