1: I checked for proper RSA base blinding, a secure CPRNG, lack of Bleichenbacher Oracles and lack of invalid curve attack vectors. It uses GMP for bignum stuff so carry propagation bugs are unlikely. There are some things that aren't super nice. The CPRNG does not reseed on forks, the included AES doesn't look particularity time constant and the library doesn't use mlock() nor zeros secrets after use.
The existing Botan C API is in fact sufficient for OpenPGP already, https://github.com/riboseinc/rnp is in C++ now but was originally C and uses Botan's C API.
But Nettle is IMO quite solid and the developer is very skilled, so full steam ahead.
Is there any relation between Sequoia and the BoringPGP spec?