Suppose we have an XMSSMT instance that is capable of signing up to 2^60 messages securely when given some state-dependent information, I could envision making it stateless as follow:
Hash the message and truncate the digest $t$ to 60 bits.
Split $t$ into $d$ (Hypertree layers) chunks and interpret as integer leaf indicies at every layer.
At each layer, each leaf signs a WOTS+ public key generated from a seed created by encrypting the leaf index.
Obviously this would reduce the security of the scheme from acceptable to merely 30 bits, because the signing key is selected based on a 60-bit hash digest.
Q1: Can this be salvaged by using some unique-mapping function? Or we have a better chance rely on UTC time or use SPHINCS-family schemes?
Q2: What benefit can we get out of using XMSSMT over SPHINCS-family schemes?