How to dialkylate a cyclohexene, and convert a carbonyl to an alkene? I'm trying to design a synthesis of D from C. All I can think of is using $\ce{Me2CuLi}$ to add the methyl group to the alkene. I thought maybe then if isobutyl bromide was in the reaction mixture when $\ce{Me2CuLi}$ pushed the negative charge onto the oxygen and it came back down that could do an $\mathrm{S_N2}$ with isobutyl bromide, but I've never seen that before. Then for converting the carbonyl to an alkene I have no idea.
You are right about using the Gilman in a 1,4-Michael addition. The enolate created from this can then be used in the nucleophilic attack of a haloalkane. Then all that's left to do is a Wittig reaction, swapping the carbonyl oxygen for a carbon. The full stepwise synthesis would be: