Is this wall above ground or in a basement? Water penetration isn't good so you need to find an inspector (home insp or mold insp) who can examine your roof, gutters, siding and who has an "thermal imaging camera". Lesley is on the mark about mold which needs moisture to grow. So, have a pro get up on your roof especially in the area near the water stains. Another tip, get it done before winter and bank on tearing out that wall to treat the wood studs with bleach to kill anything.

Add a half length horizontal mirror to the left side . It should be able to reflect the hanging light in the dining room as well. Get extendable dining table. Make sure the dining table and chairs dont obstruct the flow of traffic to the other room

By adding a mirrored wall you will double everything on its view , are you ready for the look of doubles ? Plus I think that not many people would appreciate looking at themselves in a full size mirror while setting down for dinner . If the answer is yes then go ahead ,but I would work with the space and use a smaller mirror instead .

Sorry I don't understand. With the high ceiling, would ceiling fan(s) work as well as an AC?