We're not privvy to the specific manufacturing process used by Apple's contract manufacturer. We don't know if it involves vacuum at all, or UV light (maybe the glue is hardened by heat in an oven, or using a catalyst like epoxy for example.)But 3 minutes of YouTube searches and a few minutes more watching some videos that show lamination, would reveal that the process works better on large surfaces, requires UV light hardening of the adhesive and vacuum treatment.
I fail to see your line of reasoning. Cost effectiveness is a relative measure. If it works on sapphire it will work equally well on gorilla glass. If Apple's bothering to de-burr the inside edges of the speaker housing of the aluminium case (a place you can't even see or reach without busting the watch open) using lasers - tech pretty much unheard of in consumer gear mass production - I doubt they'll balk at laminating the display either. The sport Watch is a cheaper watch, but it isn't cheap. And it's not made cheaply either. So yea, it'll have a laminated display.Since you cannot cut the rounded displays easily after doing so, I would assume that doing so for very small displays and sheets of glass is not cost effective.
Well, there's the cold forging, and the zirconia back with its inlaid sapphire windows, all polished to a few microns-level smoothness. But yeah, let's assume the display isn't laminated, because that would necessitate a second, radically different method of construction which would be thicker and less performant...Since the hardware is identical, and the price difference between stainless steel and aluminum is not really that large (considering that the milling is identical), I would think that the difference is caused by the yield of the sapphire display and the extra steps for laminating.