Suggestions and help with plumbing my tank please?

[mark]

Don't use the lower hole for the drain even with a check-valve, unless you say have a 90 then extending upward to some form of overflow. The overflow does not need to be a box going all the way down but could be a coast to coast style tying into the bulkhead or could also consider just a piece of horizontal PVC (pictures here).

Upper hole for return.