Most critically, is any kind of jailbreaking or exploitation of bugs needed to get builds not released by the manufacturer onto the device?
Yes. What's usually left out of the discussion about Android vs. iPhone's "openness" is that you have to root and unlock your Android phone to install any non-vendor approved operating system mods, just like the iPhone. Here's a good starting point if you want to do this: http://androidandme.com/2010/01/hacks/video-how-to-unlock-an...
It depends on the device and the requirements of the devices' carriers. Most carriers specify that devices has bootloaders that will only allow flashing system images signed by the carrier; these devices require some manner of exploit to gain root access and overwrite the system's bootloader. There are also devices capable of running Android that have no restrictions on the bootloader at all, such as the Neo Freerunner; almost as "open" is the Nexus One, which has a bootloader with a user-accessible method to "unlock" the phone and flash a new system image at the cost of voiding your warranty.
Strictly that's true, but it depends what you mean by "operating system mods." Apps that replace "core functions" like the dialer, home screen, browser, etc. are widely available (in the official Market and elsewhere) for Android.
Yes. What's usually left out of the discussion about Android vs. iPhone's "openness" is that you have to root and unlock your Android phone to install any non-vendor approved operating system mods, just like the iPhone. Here's a good starting point if you want to do this: http://androidandme.com/2010/01/hacks/video-how-to-unlock-an...