I'm using wolfssl in a mesh network without any issue. Openssl should be considered as much reliable. The only real difference is that wolfssl is lighter in memory, and I assume that is why it has been chosen by the dev team as default.
Because the package is in master. It was added recently. Besides, when wanting to install openvpn-easy-rsa, I see that you need OpenSSL, so you are right that this tool is not yet compatible with WolfSSL.
I think it is a misconception that users should write documentation.
They don't have the knowledge (that's why they refer to documentation), the technical means (only single devices usually) or the time to research to write comprehensive docs.
It is the developer that has all knowledge about the code and (hopefully) sufficient technical equipment to test the procedures.
Users choose solutions based on technical versatility, stability and available help to implement them.
If the documentation (at the time of writing v 21.02-rc3 is out) still covers LEDE 17.x there is something seriously flawed.
If there are how-tos out there that suggest installing batman while simple correcting the firmware and using standard functionality would suffice there is something seriously flawed.
As for me: I cannot recompile openwrt to use openssl and I cannot create binaries to correct the firmware. I do expect this from the developers of Openwrt or have to revert to the factory image of my router.
On a final note: openssl was default in 19.07. It was replaced on 21.02 and broke a lot of scripts. If these types of decision are taken so lightheartedly Openwrt will end like most open source projects.