How to deterministically generate a 256 bit ECC key from 128 bits of entropy without a CSPRNG A safe way to generate a 256 bit ECC key from 128 bits of entropy is to use a CSPRNG, according to this answer: https://crypto.stackexchange.com/a/56551/43864

However, it's difficult to find a cross language, cross platform CSPRNG that guarantees to produce the same result across all implementations.

What would be a safe alternative to using a CSPRNG? Ideally the answer would involve just the use of SHA3-256, for which implementations are widely available.
If you need a CSPRNG, there is no safe alternative.

Your assertions that “it's difficult to find a cross language, cross platform CSPRNG” and “SHA3-256, for which implementations are widely available” are contradictory. SHA3 implementations are not very common, whereas every cryptography library provides a CSPRNG. Furthermore, given a hash implementation, it is fairly easy to construct a CSPRNG: use either Hash_DRBG or HMAC_DRBG, both defined by NIST SP 800-09A. Hash_DRBG is slightly faster and HMAC_DRBG is slightly more resistant in case a weakness in the hash algorithm is found.