How can I specify connection port for Keyserver?

Add a new Keyserver (or modify existing one) by going to Preferences -> General -> Keyservers. Enter the port number after the Keyserver address and preceded it by a colon. For example, "p80.pool.sks-keyservers.net:80" (without quotation marks) means that server "p80.pool.sks-keyservers.net" is working on a port 80.

Default connection port is 11371 and it doesn't need to be specified.

A wrong primary user id is shown when searching on a Keyserver

Unfortunately, this is a bug in the SKS Keyserver software. Its machine-readable output returns the user ids in an arbitrary order. Read the related bug report for more information.

How do I activate OpenKeychain in K9-Mail?

To use OpenKeychain with K9-Mail, you want to follow these steps:

1. Open K9-Mail and long-tap on the account you want to use OpenKeychain with.
2. Select "Account settings" and scroll to the very bottom and click "Cryptography".
3. Click on "OpenPGP Provider" and select OpenKeychain from the list.

Thats it.