Using your numbering:

1)
There are several advantages to creating an account with an email: first, it's visible in the application to other users; the signup process uses a verification code to vet that you actually are the person associated with that email, which gives a pretty strong indication that that account is in fact owned by the person behind that email. (And, from there, make it simpler to verify things out of band if necessary.) Second, it allows us to contact you, in case there are any issues we need to make you aware of. (Simple example: if you have a premium account and there are any billing issues.)

You can mix both types of accounts in a team or channel; technically speaking, one just has that extra bit of verification information. However, when it comes to your opsec, there are pros and cons to both, so it's your choice whether or not you wish to register using your email or under an alias.

2)
If you need to change the Semaphor account for your system, rename Semaphor's appdata folder for your system. In Windows, it's in %appdata% under Semaphor; in OSX, it's in ~/Library/Application Support/ under Semaphor. (I don't have a linix box at hand right now to look it up directly, but I imagine it's under ~/.config/Semaphor.) On iOS or Android, purge the local data folder. Before you do anything involving appdata, if you're going to need to access that account again, make certain you have generated a recovery key and written it down somewhere and/or have saved it in a password manager! We CANNOT see your recovery key, and we have no way to generate one if you lose it.

As a minor point of order, 'logging in' and 'logging out' is not quite the right terminology, because you're not creating a session on our servers. Rather - much like the SpiderOak One backup client - the database is on your end; the server backs it up and keeps it in sync between users and devices, but your local machine searches it and alters it. It'd be more accurate to say 'uninstall old account' and 'install new account'. (I only offer that so it's clearer on which end these actions are happening.)

3)
There are some tentative designs in early planning, but it's far too early to give any estimates. On my end, I keep my work and personal accounts separate by using different computer accounts - for example user A on my desktop accesses my work Semaphor account, and user B on my desktop accesses my personal Semaphor account.

4)
There is a contact list: tap your user icon, then tap 'see all' in the list of contacts. (It's below your bio; if you don't have any contacts yet, rather than show an empty contact list, it won't be displayed at all.) If you need to see only one team's directory, go to the team options for that team and choose User directory.

When it comes to ad hoc person-to-person communication... maybe in the future, but not immediately. Right now, we're focusing on team collaboration tools because that's what we needed when we were looking for an appropriate program. What we do have in the works are ways to make joining teams nearly painless, as well as team and channel management tools, so throwing together ad-hoc teams and channels and deleting them afterwards will be far simpler.