There was a formal offer. The OP. There was a firm acceptance: “I accept your terms”. Ambiguities in a contract are construed against the drafting party. 
Sorry RNATB, but the decision has been handed down. You will have to take it up at the Appellate level. I understand that this consists of a 3 judge panel…
The OP states that the suspension will begin after a 7-day grace period. Implicit in this is that the grace period starts when the outcome of the wager is determined. In this case, that would be when the race was decided; if we want to be fussy about it, I would place that point at the end of Romney’s concession speech. WT? did not state any changes in those terms until after RNATB had formally accepted the terms in the OP.
As a Fair and Balanced (or a least Balanced) party, my judgment is that once a wager is accepted, one cannot get out of it by making an unanswered proposal to change the terms.