Their two votes alone get someone very close to the number 20 spot by themselves

No they don't. The total is about 28 billion and it takes 59 billion to get to #20 (and #20 is pretty vulnerable too, if other stakeholders don't like a #20, then votes get changed and they are easily pushed out).

it is plain to see that under the current system we could have one massive account selecting all 20 witnesses

Only Steemit itself is close. We should try to get more participation so that is no longer the case, but in any case as Steemit continues to sell off 800K every month it will soon stop being the case anyway.

All top witnesses need votes from many stakeholders in order to get voted in, not just two.

I think that many of those other votes are only there because they already received a vote from a top stakeholder

It could just as easily be argued in the other direction. Big stakeholders generally don't go fishing around for the #153 witness to vote. More typical is that backup witnesses start getting votes from the biggest stakeholders and sometimes eventually get voted into the top 20, after they have already climbed up the ladder with smaller votes. And yes there have been numerous top 20 witnesses who have done this. One second, let me make a quick count...roughly 11-12 of the current top witnesses got there by climbing up the ranks before they got votes from the biggest stakeholders. That's my informal assessment/recollection and may be off by a bit in either direction, but not so far off.