Etops is a planning exercise
I'm getting sick of hearing this, as though it doesn't matter what you do after planning is finished.
Yes, it is largely a planning exercise, but it's a planning exercise to ensure you have sufficient fuel so that you don't end up in the drink. Same as a normal flight!
If the worst emergency happens, at the worst point along the route, then you
MUST still be able to land on a piece of concrete. My operator states that, at the CP, I need to have fuel to fly to my ERA (they give me 5% contingency as well) with a fuel allowance for severe icing and APU burn, hold for 15 mins and make a landing.
My flight plan does use the variable reserve at departure as part of the fuel required, as you state.
So, to answer your question, if I have burned all my variable reserve before takeoff, I would get airborne and have a look at how the flight was progressing. As you said, if the critical CP is the last one, then you have about 8 hours to come up with an alternative plan, which may include finding a new ERA.
But, if it looked like I was to going to get to the CP with insufficient fuel to divert (as above), I would not go there: as an commercial operator of a Perf A aircraft, I have an obligation to my passengers not to put them in harm's way. I cannot knowingly fly to the CP with the full understanding that if the worst were to happen, we would end up swimming due to a lack of fuel. You would need to find another ERA or re-route or pick up some gas somewhere.