That is explained in detail in this news article on compo.sr including the migration plans and implications.

EDIT: As of now, we completed the migration to composr.app.

We do not know. Due to economical circumstances, the core developers of Composr all had to step away from heavy focus on the project to focus instead on jobs / income (Composr doesn't pay our bills). The speed at which v11 is developed and will reach stable largely depends on how much the community gets involved in development.

Yes (probably). Always remember especially during alpha and beta phases that things can break. Please make backups first. And let us know on the issue tracker if an issue occurs.
 

Note: If you are on 11 alpha 3 or before, you must first upgrade to 11 alpha 4 before you can upgrade to any of the later releases.

Yes, for basic non-custom v10 sites. However, expect some issues to arise in the process. And please report those issues to the tracker.

For v10 sites with non-bundled addons or customisations, upgrading to v11 will likely break the site until you update the non-bundled addons or temporarily remove the customisations.

We aim to have the upgrader fully functional for a large majority of types of sites and for sites using non-bundled addons by the time version 11 reaches release candidate stage.

We do not recommend upgrading production v10 sites at this time until v11 reaches stable.

Absolutely! Frequent changes may be made to both bundled and non-bundled addons. Bundled addons will be updated via the upgrader. For non-bundled addons, you will have to update them through the addons management screen (Admin Zone > Structure > Addons). You will be informed which ones need updated.

Once version 11 reaches release candidate, we do not expect frequent updates to non-bundled addons anymore. But it is still good practice to check after every update.

It is not recommended you do this because you may run into issues. For example, some changes were made in crypto_master.php and data/upgrader2.php prior to the first v11 alpha. These changes will result in a broken upgrade unless you already pulled these changes from git. Similar situations could happen in the future as well.

It is recommended you either discard the git install and use a fresh install from the Downloads, or to maintain a git install and a bleeding-edge release install separately. Do not combine the two.

That depends on several factors. Here are some pros and cons of each: