I did and I LOVE it. Mine was more out of frustration than room size or natural light. Our living room front wall, the front door, and the dining room front wall are all one in the same (with no separation). Instead of having a color change somewhere in the middle, we finally painted it all one color (Shaker Beige). We then painted our 3 remaining dining room walls Shaved Chocolate (yummy lol) and our 3 remaining living room walls Olive Leaf (I think). It was the best solution to our problem and looks great, imo. And I agree with Mitchdesj, what your eye tells you. Once I finally started trusting myself and others here, I have never been happier.

It's ok to mix primers, but if you are using the latex version of Kilz, that's not going to block odors really or stains. But, in my experience, washing the nicotine/tar stains is usually good enough unless the smoker really smoked a ton in the house. Also, you really need to rinse that TSP well before painting or that may cause some adhesion issues. If you are buying the BIN, what you want is the shellac based stuff, not the synthetic.

No, you are confused. But this is not your fault. It is the fault of the industry. There is a difference between dead flat and washable flat. If you are interested in using a dead flat paint, one that has no washability, then they are all pretty much the same and sometimes it is better to use a cheaper version. Cheap flat paints are filled with cheap ingredients like chalk and clay, It is these cheap ingredients that make the dried paint film look good in critical light (no flashing, roller lines, etc) and also helps to make the paint easier to touch up. However, because of the cheap ingredients, the paint film has no integrity at all, which means if you touch it or bump into it, there will be a dead spot or a dull spot. This is why we like to use it on ceilings. It looks great and nobody will touch a ceiling. A washable flat is a different animal entirely. It's not a flat paint at all, but the manufacturers will call it a flat anyway because they are stupid. A washable flat should be treated like an eggshell or a semi gloss or any paint that forms a protective skin on top of the paint film (enamel). The best system for these types of washable paints is a quality primer followed by 2 topcoats. The problem is that it can be hard to tell when you are shopping for paint which paints are dead flats and which paints are washable flats. It's even more confusing when manufacturers of dead flat paints are putting the word 'washable' or 'cleanable' or 'scrubbable' on their cans of dead flat paints! So, if you come on a forum asking for advice about flat paint, we assume you are talking about dead flat. If you want advice about washable flat paint, then you should have specified.

Two coats. Go into the KM store and check out their standard offwhites and see if any work. It is easier to do touchups in the future if it's one of the stocked colors, and if no color in particular calls out to you, who not make it easy? (I've used KM paint but not in offwhite. I did do custom kitchen cabinets in KM's "Swiss Coffee" because that's what the previous owners had used on the trim in the house, but that's white.)