I recommend using git over SVN. Easier branching and merging means that all of these points will go more smoothly for you.

Applying upgrades: Do this in dev. Make a branch (this is where git really shines), apply the patch files or even better, unpack a new Magento version and point it to your old database. No extensions yet. Open the admin in the new Magento installation an hope for the best. Upgrading between minor versions probably won't be a problem. You will probably have to reindex after all the new stuff installs. Do a commit once this is stable, then gradually bring into the branch your extensions and themes, make any code adjustments, then doing a commit after each step proves stable.

Environment-dependent files: .htaccess and app/etc/local.xml. I do a separate version for each: local.dev.xml, htaccess-dev local.staging.xml, htaccess-staging local.production.xml, htaccess-production

...and then make softlinks to them for each environment:

ln -s htaccess-dev .htaccess cd app/etc/ ln -s local.dev.xml local.xml

and so on.

Restricting access to certain developers: I don't do this. However, you can develop a deploy strategy in git that lets a release manager decide what goes in and what doesn't.

Managing database changes: That's the trickiest part. We just use mysqldump from production, and have some ready-made "env-setup.sql" files for each environment. Something like this (your ids may vary):

UPDATE core_config_data SET value='http://magento.dev/' WHERE config_id IN (3,4);

I usually add some more instructions that will change payment gateways over to test environments, change outgoing emails, etc. Most of these you will find in core_config_data.

Remember that modules will usually make their own changes to the db, so applying a well-made module usually takes care of itself. In any case, never apply untested changes to prod, always do "rehearsals" on local and staging environments.

You can get the CMS (pages and static blocks) data out of the database by dumping and loading just the cms_* tables from whatever environment development was done on.

Good luck!