What you are missing it that if the fork is actually made, blocks compatible with the 20 MB fork only will come after 75% of the nodes have announced themselves that they will be ready for the fork. So, either this happens and it's more profitable to mine on the 20 MB chain, or this doesn't happen and nobody will be mining on the 20 MB chain.
First of all, the prices can start diverging before the fork actually happens. The economic resolution will probably not need an actual fork to happen.
In addition to that, even if only 25% of miners remain on bitcoin at first, there will still be one block every 40 minutes, and one block every 13 minutes and 20 seconds on the 75% chain which seems more than enough to get some trading going on.
Now, in addition to this, you could perfectly imagine that some miners decided to broadcast v4-tagged blocks before the fork, for the sole purpose of inflating the gavincoin/bitcoin rate, with no intention of actually honoring the hard-fork. By doing this they would lead unsuspecting bagholders to sell more bitcoin for gavincoin than they would have otherwise. If this happens, god help you, you'll think you'll have 75% of the miner support, but when the hard fork comes you'll find all this hashrate disappearing from under your feet and supporting the other chain...
"We heard you like market manipulation, so we put some market manipulation in your market manipulation..."