You are wrong, there are no widely used forks of Thunderbird AFAIK. Thunderbird is based on Mozilla and has a huge codebase that is very hard to maintain. All other popular email clients have totally different code and based on other libraries. They can be similar in how they appear, but not in what bugs they have.
It is not the question of funding. Thunderbird has always had a number of long standing bugs. Speaking about such rare use cases, I don't think someone care about them. Anyway, I recommend using software that I know it worked correctly, not that worked incorrectly and could be fixed but requires further testing.
Systems with bash but without standard POSIX utils? I know some without bash (freebsd by default, busybox based distros etc.) and with grep, sed and awk, but not vice versa.
It is the POP3 workflow, not IMAP. Maybe setup your client to use POP3 and remove mails from server after receiving? However I don't recommend Thunderbird, its POP3 support was very buggy when I used it (many years ago). Try Sylpheed or Claws Mail, for example.
When piping output of find to xargs, always use -print0 option of find and -0 option of xargs. This allows processing files with any allowed characters in names (spaces, new lines etc.). (However I prefer -exec.)
There's an i command to insert a line in sed, it is better to use it instead of s/^/...\n/. It makes code more readable (if we can talk about readability of sed code, huh).
If you want to split a delimiter separated line and print some field, you need cut. Keep awk for more complicated tasks.
In most cases extended POSIX regexes are enough and looks the same as perl regexes.
I also used perl until I needed to write highly portable scripts that can be run on systems without perl interpreter (e.g. some minimal linux containers). Simple things are also simple to do with grep/sed/awk, more complex things can be done with awk but require a longer code in comparison with perl.
First of all, check that hardware virtualization is enabled in your UEFI setup.
You need to have packages virtualbox-dkms and linux-headers installed (together with all their dependencies, but you don't have to check them manually). linux-headers must be of the same version as linux-image. That's all that you need to get driver properly built. Don't listen those users who recommend you to remove and reinstall these packages or install additional packages manually, this kind of magic doesn't work.
Also note that you cannot use kvm and virtualbox simultaneously. If you are using some kvm-based virtualization system, it causes a conflict.
Well, reference manual was enough for me. There' nothing difficult. You probably have troubles with your lua code, try executing it in the standalone interpreter.
Well, importing settings from one manager to another is also a problem. It can be solved with a smple script, but you need to learn structure of both configs to write it. Or try to search on github, maybe someone has already solved this.
A tabbed list can be replaced with shell autocompletion. If you name servers in config like _group_subgroup_server, you only need to type few characters and hit Tab to find a server you need.
Did you read the gcc documentation? Don't you think it is already overcomplicated?