Regarding $200 for $100 certificates: they are only available on Viking river cruises, but can be redeemed on any new Viking cruise (1 redeemed certificate per person). The purchase limit on our Rhine cruise last month was 10 certificates per person. BTW, these certificates are transferable and never expire- they can be combined with other discounts.
The only possible way I can think of to use certificates would be to cancel and rebook your September cruise. Not sure if this can be done with the on board Viking agent. Also, any perks other than Explorer society discount would be lost if you rebook. Similarly, I believe you would have to cancel and rebook within 14 days to get the welcome back SBC. I also assume you’re paying cash (not using a future cruise voucher from a cancellation). I’m under the impression that only a Viking agent can do the welcome back credit.