I'm looking for some advice on hooking up a new Noritz NRC-111 to an existing 40 gallon gas-fired tank. Effiency and ROI are not an issue. Based on the goals below, can you advise as to which of the following scenarios may be viable?

Background Info:
-40 gallon gas tank heater.
-Noritz NRC-111 tankless.
-Each heater will be plumbed to be able to by bypassed.
-Located in SE Michigan, winter incoming water temp around 45 degrees.
-Two showers, one large soaking tub (approx. 90 gal.), high efficiency dishwasher, front loading washer).

Goals (in no particular order):
1. Maximize hot water flow to allow as much hot water use as possible.
2. Eliminate issues with cold-water sandwiching.
3. Eliminate minimum flow requirements (for high efficiency dishwasher, front loading washer).
4. Maintain the amount of time it takes to get hot water (compared to tank).
5. Have a backup option for hot water in the event either the tank or the tankless fail or needs maintenance.

Scenarios:
A. Hook-up the existing tank in series to the output of the Noritz and use it as a buffer tank. Set the tank to maintain at five degrees below output of Noritz. Pros: Meets goals 2,3, 4 and 5. Cons: Doesn't meet goal 1, flow may be limited at Noritz based on incoming water temperature.

B. Hook-up the existing tank in series to the input of the Noritz and use it to pre-heat the incoming water. Set the tank to maintain at 70 degrees. Pros: Meets goal 1 and 5. Cons: Doesn't meet goals 2, 3 and 4. Once the tank runs out, the incoming water temperature goes to the Noritz direct and the tank might as well not exist.

C. Hook-up the existing tank in parallel to the Noritz. Shut-off flow to input and output of tank. Drain tank and have available as backup in case tankless fails/needs maintenance. Pros: Meets goal 5. Cons: Doesn't meet goals 1, 2, 3 or 4. Could the tank corrode with just air inside?

D. Scenario B with a small electric heater in series to the output of the Noritz as a buffer tank. Pros: Meets all goals. Cons: Once the tank runs out, the incoming water temperature goes to the Noritz direct and the tank might as well not exist.

Thoughts?

Thanks in advance!
Rich