If they all want to play the table of seven then it will be a cake walk. Even with a table of five optimised PCs in the same APL quagmire would cakewalk it. Happens at my tables on a regular basis.
As experienced players they need to deal with that fact or one of them steps up to GM. If they don't want to GM this one cold, suggest running an evergreen with the table the gets bumped and then arrange for that scenario to run a second time, preferably by one of those that got to play it the first go around.
As the GM it is your call and not theirs to seat seven players. As experienced players it's their role to understand the realities of the situation. And please don't ever, under any circcmstance, rewrite the scenario to suit the table. That is not what PFS is about, runs contrary to the guide and intent of the organised play program. This kind of stuff gives Venture Captains migraines.