How complex can bitcoin multi-signature addresses be?
The limits of such constructs are the 520 bytes of the redeem script (its opcodes). A signature is roughly 70 Bytes (length code, r-key, s-key ...), so you can calculate what n-of-m versions fit into 520 bytes.
All your ideas are possible with bitcoin.

E.g. you can create a 2 out of 6 multisig for B,C,D,E,F and G.

When you say "A" can sign it, I believe it is the same as "H". 
Then you run into 8 keys (8x70=560 bam...), so you are beyond the limits. 
You might circumvent this when creating a simple "smart contract":

if &lt;pubkey A&gt; OR &lt;pubkey B&gt; else &lt;2-of-6 msig&gt;


then you are again in the limits. These type of ideas are explained e.g. in Andreas' book "Mastering Bitcoin, 2nd edition", chapter 7 "Advanced Transactions and Scripting".