So CDMA scales beautifully with number of phones, while GSM does not scale at all. Consequently the CDMA carriers were the first to roll out 2g service. There was no way to fix GSM for data. They had to add on a different standard for data, which most carriers implemented with CDMA or wideband CDMA. That's right, the HSDPA data service on most 3g GSM phones was actually CDMA. That's why you could browse the web and talk on a GSM phone at the same time - it had one TDMA radio for voice, and a second CDMA radio for data. CDMA phones couldn't do that (unless they supported voice over IP) because they only had one CDMA radio for both.
As someone who developed GPRS for Ericsson back in the day, I don't even know where to start...
There were a number of different competing standards, in different parts of the world. That CDMA wasn't mandated in the US was not for lack of trying by the US manufacturers.
And, no, if we're talking about true packet data, i.e. not "phone modems", GSM/GPRS did emphatically not use a dedicated slot per user for data communication. Instead all the available "data" slots (and there can be many) were/are shared dynamically between all the users wanting to receive/transmit using dynamic reservation protocols (depending on, among other things, whether you have data to send/receive). Indeed EDGE is just GSM/GPRS with more data slots available, and with mobiles that can use more slots in sequence.
All this is moot anyway, as the explosion of demand for mobile IP, necessitated completely new systems anyway. And since they were new, they weren't hampered by what was already there. You say that UMTS is based on CDMA, which is true, but there are also FDMA and TDMA parts, and even versions of the UMTS protocols. So that UMTS is CDMA and that's superior to GSM which is TDMA does not follow.
I could write a book about the rest, but that'll have to do for now.