Please see here: https://www.davdroid.com/manual/accounts/#c272
DAVdroid address books accounts: They’re named after the CardDAV address book which they are connected to, plus a hash of the URL, for instance: My addresses (UA). The hash is necessary when there are two address books with the same name. The address book account names will be shown in your contacts app as possible destinations for contacts.
and here: https://forums.bitfire.at/topic/1432/what-does-cq-at-the-end-of-my-e-mail-address-stand-for
Without the hash (the two letters), it would not be possible to have two address books with the same name (e.g. “Address book”) in two different CardDAV accounts.
Duplicate of #3, please follow up there.
I have recently seen a certificate without CA flag that could be imported and used with DAVdroid in Android 4.4, so when I find time, there might be a CAdroid update that allows certificates without CA flag to be imported in certain Android versions. That would greatly improve user experience for DAVdroid users, too.
For more information, please see in #3, https://davdroid.bitfire.at/faq/entry/importing-a-certificate, and http://cadroid.bitfire.at/.