0. When is the step of building flashable image from these packages?
i. e. completely free maemo image, without proprietary nokia packages. And then we'll see how it looks and what can be done, what needs to be rewritten.

1. If I upgrade using your repository, and device get bricked I have no idea how to check what's wrong, which package causes that. If something breaks, but device boots, and it's possible to open a terminal, then yes, something could be done to investigate the problem.

2. Most important. Anyway, I think you are very close to creation of flashable image. And not only for n900, because n900 is a dying hardware, but for any modern device, which allows booting non vendor made operating system. Of course you probably have to start with n900. Ask here or in irc about how to create the image.
I believe you should follow mer strategy and separate the kernel (hardware adaptation) from the rest of the system. So that you can build all maemo packages for different architectures, and in the same time, when preparing an installable image, at this step you unite those packages with vendor specific kernel / hardware adaptation, which is usually non-free because it contains non-free software, drivers, and usually we can do nothing about it.

3. Even active n810 users will benefit from that. For instance, I use n810 every day, and nokia did not provide fremantle image for n810, I believe mostly for marketing purposes. Openmoko community will benefit from that - they could run fremantle like operating system on their devices. Nexus users could benefit, they could run canonical GNU/Linux system instead of their inferior, sorry, androids.