I recommend a dedicated account*, but you can also use it with your existing account in parallel with an other MUA/email client.
As long as DC runs in the background it can move all DC emails into the DeltaChat imap folder.
If it can’t run always in the background, the chat messages stay in the inbox, but you can also solve this with server-side filter rules (move all mails with the chat header to the DeltaChat as soon as they arrive)
If you can’t do server rules, I believe a filter rule in thunderbird also works.
Autocrypt, all Email clients capable of the Autocrypt standard can decrypt the emails.
But the whole encryption is opportunistic, so as soon as the other side has the key it does encrypt - which means the message until the first answer are not encrypted. Unless DC already knows the key because of a shared group.
There is an autocrypt plugin for TB, but as far as I know it doesn’t work on the new thunderbird 78 that broke all addons, but they (TB seem to work on basic autocrypt support integrated directly into TB.