You spread your 7 votes from the 73 million SP across an overlapping mix of 17 witnesses, which is 30 million each. That's probably more than enough. Currently top witnesses get around 45 million but that is with everyone having 30 votes as well. If you drop the 30 to 7, then the other witnesses' vote totals will drop too. If we, probably conservatively, estimate that totals drop by half, then totals are down to 22 million, and 30 million is easily in control.

If somehow that isn't enough, which I doubt, then you buy a small amount of STEEM to net up the total, campaign and count on at least a few people always being easily swayed to anything or confused, and/or you offer a few people to pay for their votes. It won't be hard to get what you need, even if this means the rest of the stakeholders are voting, say, 90% against you.

No plausible voting rules are going to contain 71% of the active voting stake from taking control. It won't happen. (And if you could, it would greatly weaken the chain by allowing small minority, potentially attacker, stakes to exert greater influence).