Backwater valves, furnace drains, and vent pipes
1) Recommendations vary from inspecting every 3 months to every 1 year. You're checking for any debris that might prevent the valve from closing if water does come up the drain backwards. If a furnace is on top of the access point, you won't be able to inspect it.

2) The vent pipe must be within a certain distance of the drain trap, determined by the size of the drain pipe. Section 906.1 of the IPC states that for a 2" pipe, your vent needs to be located within 8 feet of the trap. Just eyeballing your setup, it looks like it's possibly OK. However, your locale may have adopted modifications to the code, so you'll need to check with your inspector to ensure that it meets code. 

That said, please make sure that your inspector is aware of the changes. The plumbing had to be approved before the slab was poured, and the HVAC contractor knew it wasn't going to get a second look. If the inspector gets another chance to review it, changes may be required. 

Good luck!