If I am proper, there are no less than two strategies for miners to embrace a reference hash to a merge mined block within the coinbase transaction:

1. Dedication hash in an OP_RETURN out put within the coinbase transaction (a la RSK)
2. Dedication hash within the enter script sig of the coinbase transaction (a la Namecoin)

A. Which one is preferable from an entire system perspective, and from the perspective of a person miner?

There may be additionally a specification for merklising a number of merge mined cash into one hash. It has the next observe:

NOTE: This commonplace is utilized by Namecoin, however new merged mining information
ought to seemingly suggest a brand new BIP to supercede it with one thing primarily based
on p2pool’s merged mining.

B. What’s the newest specification, or extensively accepted observe, for merged mining?

C. Are there any apparent enhancements/fixes new merge mined methods may add?

Lastly,

D. are there another strategies used for merged mining?

See additionally:

BIP301 (draft) Blind merge mining proposal

How does merged mining work?