OK, let's see:
I use two Thunderbird profiles but only one shows the p=p addon.
Since TB 68, TB has the "profile per install" feature, that means that it distinguishes between regular release, beta and Daily installations. Each installation has one or more dedicated profiles, and it's not advisable to run a - say - beta installation in a release profile. Let's assume that you only have a regular release installation of 78. You can still have two or more profiles, one being the default for the installation. One Mac, which you have, we only install into the last used profile. On Windows, it's different, we present a list of all default profiles if there are multiple installations.
How to install into the second profile: Open the profile folder of the profile where pEp was installed via "Help > Troubleshooting Information", Profile folder, Open Folder. In there is a folder "extensions" and in there you'll see the pEp add-on, it's called pEp4Tb@pEp.security.xpi. Start TB on the other profile and install the add-on from file. Sorry about this clumsy install, our market strategists were of the opinion that Mac users only have one install and one profile.
emails with subject “p≡p key management message - please ignore”
What is the folder "pEp" (sub folder of the Inbox) for ? Can I delete it ?
It has also happened that a number of them remained in the Inbox from where I deleted them.
These e-mail are used for synchronising between multiple devices. That's called "pEp Sync". You can globally disable pEp Sync, or you can disable it per TB identity. Please check the pEp Options which you can find under Tools or in the Hamburger/Application menu. After arriving in the inbox, Sync messages are moved to the pEp folder. Please don't delete that folder. A clean-up happens every 10 minutes and older message are removed from the pEp folder. They are also removed from the inbox. If they don't get removed from the inbox, then the inbox needs a repair since its index is corrupt. To repair the inbox, right-click, select "Properties" from the context menu, then "Repair Folder". For an IMAP folder, that will likely re-download all messages. You can also remove those messages manually.
A number draft emails cannot be decrypted even though I do not remember encrypting them
Sadly, once you enable OpenPGP in TB, all drafts are encrypted. There has been some discussion in the TB community to make draft encryption optionally like it was in Enigmail, but I don't know whether or when that will be implemented, see bugzilla.mozilla.org/show_bug.cgi?id=1672047. pEp doesn't have that problem since it has the concept of a "trusted server". So if you store your drafts on a trusted server, or in Local Folders, which you can set to "trusted", then drafts are not encrypted. Also see bugzilla.mozilla.org/show_bug.cgi?id=1650551#c28
If after installing pEp for Thunderbird something couldn't be decrypted, then the installation didn't import your private key(s).
Here some background. Enigmail in TB 68 or earlier used GnuPG as a backend. The OpenPGP implemetation in TB uses the different backend called RNP. So all that Enigmail for TB 78 does is migrate the keys from GnuPG to RNP.
The installation of pEp migrates keys from GnuPG to pEp's own storage. On Mac, that's in your home directory in a subdirectory .pep. The database consists of keys and management DB files. You can also import you keys manually, that works for private and public keys.
So I suggest the following: Delete the .pep directory (I hope it's not locked, not a Mac expert) and restart the Mac. That should give you a fresh start. Then install pEp for Thunderbird again. That should migrate all the keys from GnuPG, but not any keys you might have created in TB using OpenPGP. If you've always used the same private key, that key should have been imported. Or you can import it manuall.
In the p=p preferences under the compatibility tab, once a file has been selected for import, the field can't be cleared anymore.
Yes, we'll look into that in a future version. However, use the browse button to select another key to import. Clearing the field would just be a cosmetic option.
EDIT: It's s file picker field and the content can't be cleared. Check the "new mail sound" in TB, that also can't be cleared. If you find it totally annoying, we could clear the related preference on every restart.
tutorial states that trustwords should be compared by phone, but I'd surmise that's not a very secure way ...
Will refer that to the product designers.
The link https://www.pep.security/docs/thunderbird on https://www.pep.security/en/faq/ is incorrect, ...
Will refer that to our web designers. It's the:
your question might be covered by our user manual which can be found at https://www.pep.security/docs/thunderbird