OP needs to take personal responsibility.
It is not hard to complete basic addition and subtraction. It should have been simple to used the MQD tracker to see what the deficiency was, then look at the ticket receipt to make sure that amount is more than exceeded in base fare.
In addition, the offer to purchase status clearly states that it extends the qualification period. There is NO such disclaimer or note for this to happen with actual flights flown - those have never, ever, ever been backdated by any frequent flyer program. OP tried to apply the T&Cs from a status promotion to regular status qualification, and that's just not how things work.
If OP cannot figure out these basic tenets of qualification I worry to think about how well she/he may or may not make use of the associated benefits with Silver (aka will they act like an overentitled DM or like someone without any status and knowledge of a free bag, SDC, etc).
I'm usually on the customers side but I don't see how DL erred here. Here's how they may have erred: rounding aside, it's possible MQDs for 2017 travel was misallocated.
$2 isn't hard to find. AND there isn't really a time limit (except a point after which 2017 earned status expires). If OP goes back and compares all 2017 receipts against MQDs, $2 may be found. OP can then call DL and ask for reprocessing of that particular ticket or segment and that could fix the problem. I've had DL and even hotels forget to post 1 or 2 nights or whatever here or there that made the difference to status. The only way around it is to reconcile your account to find any differences.