Steve
I'm not part of the core team but I do a lot of DNN development, and I think you've probably got some problem in your setup that is causing the problems, because I've done a lot of DNN installs and haven't encountered the issues you've raised. My response to your points is:
1) Without this it's not easy for users with limited access to hosting to run the installs and upgrades. Very few people on shared hosting get the ability to execute scripts or packages on a server. Most can only FTP up files and then browse their server. If you take backups of everything (database and files) before upgrading you'll rarely get into a situation you cannot get out of. I would argue that the reverse is a bad idea : a framework that can't be installed via the browser would only be useful for people with their own webserver, which is a very small percentage of the userbase.
2) You shouldn't get a successful message from a unsuccesful install. I agree that a 'failed' message shoudl probably be a bit more highlighted. If your module doesn't install due to either a bad file copy or a failed Sql script execution, then you won't get a successful message.
3) All good modules have an uninstall script for the database tables, and this is run upon uninstall. Problems only arise when modules used shared files (a bad idea) and they don't have a complete Sql uninstall script. You can also manually run the uninstall script by loading it into the 'host->sql' page. This will allow you to uninstall any particular module from a database point of view, if for some reason removing the module didn't work.
4) The documentation point is valid, and I believe a concerted effort is in the works. Documenting is an unglamorous and thankless job and the volunteers are few and far between. You will find me compliling cool new tools and modules for DNN, you won't find me writing documentation. Most of the community will be the same, it's the nature of a volunteer effort. I agree it needs to be sorted out somehow, I don't have the answers.
I'm sure others will chime in, but with DNN you've got to remember it's a platform for a wide audience, and if you're coming from an IT corporate point of view somethings will feel a bit alien at first.