Hi, if you have a photo and dimension of your dining area it would help for us to see what scale of opening works best in your situation.

I agree with divadeva. More lights. If you can add pot lights do it. If they're already there add task lighting and up the wattage on the bulbs on the pot lights (replace with LED and you can make them brighter because the wattage on LED is lower for brighter light). If you have all of that add decorative lighting. If ventilation is an issue find a way to move the air around or add air filtration. Maybe the HVAC is deficient. Maybe you can add a HEPA filter instead of the standard one that comes with the unit. Call an HVAC specialist to see if there is a way to boost air flow in the room.

pooja should not share wall with toilet and should face east. Rest is well designed to suit your requirements in the footprint. A large skylight above the stairwell will bring in more light. Full vastu compliance is not possible in limited city space and for practical reasons. For example, the grounding aspect of vastu is to be one with nature and do the least harm while building. Are we doing that?

Just because you want to use the space below the stairs, do not just add drawers or shelfs unless you reaalllyyy need it. I would suggest go for adding something green to your house. Its healthy by adding air purifying plants. These plants are nowadays available in most of the nurseries and moreover Green patch in the house gives a beautiful look to the house and is also good for eyes ( the green color). Below are few picture to motivate you for Green patch or Indoor garden.