1. The LDAP integration is fairly rudimentary and not well supported. Most users who are interested in LDAP see it as a potential for single sign-on, but it won't do that -- it will only allow OJS to share user databases with other systems, but users will need to verify those credentials with OJS separately of other login systems. For single sign-on, Shibboleth is probably something better to investigate -- however, Shibboleth support was a 3rd-party contribution to OJS so unfortunately we aren't able to provide direct support and trouble-shooting. If you've got a particular problem to solve, describe it and I'll see what I can suggest.
2. The database and OJS don't have to reside on the same server; you can enter a hostname for the database connection and have it live elsewhere transparently.
3. It's hard to say anything definitive about storage requirements, since each journal varies -- but the most space-consuming part of the bargain is the file storage, so avoiding large images should keep your storage space requirements very manageable. See e.g. this discussion
4. Each journal shares the same database. There are minimal but non-zero areas of overlap; for example, a user can be an Editor in one journal, an Author in another, and have no activities in a third, but the account itself (e.g. name, email address, etc.) will be the same.
5. Yes, it's possible to mix domain names for journals within the same installation. This will require some configuration both in OJS (the base_url[...] parameters in config.inc.php) and Apache rewriting.
6. Releases are scheduled as needed to keep a dependable, stable version available (based upon the accumulation of "recommended patches", or common bugfixes against the last stable release) plus the various new pieces of functionality that we need to release, based on grants, subprojects, partnerships, and the like. You can see our current set of milestones along with more detailed descriptions of each release line here
7. A test server is not usually necessary, but of course the more development or experimentation you do, the more convenient it'll be.
Public Knowledge Project Team