a) Nokia thought they could not differentiate enough with Android so they did not choose it. Then they said they will not customize the WP7 experience - so they would essentially differentiate on hardware/pricing alone. No reason they could not do the same with Android.
b) To let go of a significant capability such as developing their own smart phone OSes and UIs - however unsuccessful they may be in the market - and betting your business on an as-yet under performing platform takes some amount of stupidity.
c) Most of what Nokia sells today are not your typical smart phones. They are losing ground to people going Android and dumping their Nokia feature phones. But they still do not seem to have a "different" strategy for low end devices targeting the developed world and dumb phone users in developed world. WP7 is not it AFAICS. Both Google ($85 dual core Broadcom SoC) and Apple (with the recent rumors of half-iPhone for sub $200 unsubsidized) have a solid plan in place to attack that segment. It is a glaring omission from Nokia and WP7 strategy - especially so since that is an under explored market and they have the highest chance of succeeding. Even if they hope dumb phone users will somehow upgrade to premium WP7 handsets - they are still late to the game.
c) Nokia's competitors who have some WP7 footprint today - how they will react tomorrow? Microsoft hasn't made any special deals with HTC/Samsung for example and their special treatment to Nokia is so close enough to them buying and running Nokia that those guys are going to be forced to rethink their WP7 strategies unless Microsoft offers them something to level the playing field.
d) Microsoft paying them money sounds good in the near term but it comes at a cost of Nokia crippling themselves and risking a whole lot on Windows Phone 7 and Microsoft. The dependence on Microsoft is a huge deal - HP could've done that but they thought it was wiser to buy Palm and bet on their own capability.
e) Why was it so hard to execute? Elop lost a golden chance of understanding this and fixing it. They won't have to execute on OS development now but still there is hardware and software integration work to do that requires lot of execution - they aren't simply going to dump hardware on Microsoft and let them deal with it. It wouldn't scale if Samsung/HTC and others expect the same.