Whew! For a minute there I thought you had some kind of late model variant to the 512Ke!
As Tom Lee correctly points out, Jobs always intended the Mac to be fan-free ... it is the engineers who attempted to defy him with expandable logic-boards boards and fans ...
As for the top vents, I don't believe it was ever a last minute decision as Jobs always intended the Macintosh to be convection cooled and fan-less. I would need to double check, but I'm pretty sure that's what I recall from various sources. Actually a lot of thought went into the deign by Apple engineers to make Job's vision possible. The top vents are one such example. Jerry Manock, responsible for the overall industrial design language at Apple up to that point, as well as specifically the rear bucket, took great pains to come up with a design which would both adequately vent the heat (notice I did not say optimally), but also keep small children (present in the households of the target consumer) from sticking things into the top of the Mac which could easily come in contact with lethal electronics. If a child stuck a paperclip into the bottom grille, at worst it would short your logicboard. But nobody would die. All this is explained in Kunkle's Apple Design and other sources. My guess for the vent inserts, versus moulded directly into case is that it was too complicated a mould for the technology of the day to do in one piece ... it is also the only aspect of the Macintosh that doesn't conform to Jobs' other mandate that the Macintosh be as close to a single moulded unit as possible with no seams. Obviously he compromised his vision for safety. You'll note the vents in the chamfered corners are equally resistant to inserting something, but were obviously accommodated by the mould.