The IIgs came out in 1988, four years after the Macintosh. As such, GS/OS was pretty heavily influenced by the Mac user interface; it was practically a demake. It still had some unique quirks, though -- for instance, it used proportional scrollbars, which wouldn't show up on the Macintosh until 1997 (in Mac OS 8).
Begging pardon, 1986, two years after the Mac. It also beat the Mac to market with a color GUI and ADB keyboards and mice. But yup, hardware-wise it was kneecapped from the start (1MHz IO bus, 65816 cpu would never rival m68k, low cpu clock speed) entirely for marketing reasons.
Yup, the built-in graphics RAM was 1MHz as well; It couldn't redraw its entire screen inside a single refresh (although by remapping the 65816 stack to screen RAM, it got close enough to do most of it). The integrated disk controller (IWM) also ran at 1MHz.
Yes, if the Mac never existed then the //GS and GS/OS would have looked quite different but they still would have existed in some form. With no Mac upgrade path, it's likely that all the different Apple II form factors would have been upgraded with the 65816 to give the platform more life. Then a next generation "Apple IV" system might have combined a 68K and 65816 to offer 32-bit performance and Apple II backwards compatibility like the PS2 having a PS1 built in. (Apologies rbanffy, I wrote this before seeing that you posted almost the same idea earlier.)
