1. Can we reserve more than one bit of the number i for different kind of derivations, not only the highest bit? This would allow addition other kinds of derivations or tweaks of existing ones in the future.
Adding more flag bits is certainly possible, but we can extend the length of the serialized i value in that case, so the only thing that needs to change is the serialization format.
5. It is certainly possible to do everything with HMAC-SHA256 alone. For instance, if you need two values like (I_L,I_R) you can do I_L=HMAC(secret,0) and I_L=HMAC(secret,1). The question is, does it reduce dependencies of the code, code review, etc. to be worthwhile?
All this might get implemented on smardcards one day. I really don't understand why you'd want to rely on a new, second hash function here, SHA512. Entropy is _not_ a reason.
I like the simplicity of a single existing construct that operates natively on 512 bits. Yes, separate SHA256 calls would work too, but are less elegant IMHO. I don't mean to say it would be less secure, and this is just bike shedding.
If smartcards have a problem with SHA512, they'll have a huge problem with ECDSA.
And third, can you change
I = HMAC( cpar, Kpar || i)
in the public derivation to
I = HMAC( HMAC(cpar,i), Kpar)
? I see the provability of the link as a quite an important property (cf. #228).
BTW, the function I will propose for payer-derived payment addresses (which is basically just a deterministic wallet of the payee in the hands of the payer) will be
K_derived := HMAC( m, K_base) *G + K_base
where m is the (hash of the) invoice or contract that is being paid for.
This would fit nicely together, as it would be the exact same function just with m substituted for HMAC(cpar,i). We could re-use code as well as security reasoning.
That provability argument is interesting, but can you come up with use cases? I'm also interested in knowing the complexity of iddo's zero-knowledge proofs that could avoid this.
