I love what you've done here and it sounds like you have the knowledge and Skills to execute your vision perfectly, whether with a hand crafted piece or a thrifted treasure. I think anything you come up with will be really good, judging by your other choices.

The only suggestion I have is that it shouldn't be any higher than the table you have there, or it will compete with your beautiful table and chairs.

Are you sure you want the chairs to slide under? How much room is there behind the chairs, if you create a 12" piece? At 19", it looks like sufficient pushback room so I don't know that having the chairs slide under the new piece is necessary. It also looks like the table could move forward an inch or so and be centered on the window, but it's hard to tell with the photo angle.

Here I am telling you whatever you do will be great, then second guessing your thought process...lol

a side board table that is narrow such as

I don't see the chairs sliding under a mantle?

This is from Hayneedle and is available in a 48" length. It's 10" deep and has a hidden drawer. It's available unfinished so you can stain it any color you wish. https://www.hayneedle.com/product/pearlmantelsabingdonfireplacemantelshelfwithsecretdrawer.cfm?ltype=child&tid=PRL125-4

I think if you want to go the mantle look you might go with some professionally installed floating shelving in wood. That way they would hold some weight vs. the ones we can buy and put up on our own only will hold 8 pounds or so.

I'd stagger them a bit and put items on them not cluttered, yet enough to make them pretty for a dining room: small candles in glass, topiary, etc.....

That's the interpretation I decided on, on my initial reading :)

Someone posted these on a different dilemma. If you have books, this could be an unexpected option.

http://www.umbra.com/cad/wall-decor/office-shelving/conceal-floating-shelf-collection

You could hang a few at the same level (so the books end up level, that is) and use that as your surface to display a vase of flowers, leaning mirror, or something else simple. Or you could stagger them like in the photo on the website, and they would add some depth while being very low profile.

Something about 12" wide should give you the extra room you need to pull out a chair comfortably. I have the same issue-my new dining room is about a foot narrower than my old. I have a buffet that I refuse to give up so I just slide the table off center at holidays to make it more comfortable. We do what we have to for the people and things we love!

Hmm...really tight...

Even if you did 12" wide, would that be wide enough for dessert trays? I know our serious desserts would not fit on such a small space.

Slightly off-beat suggestion: what about demilune tables pushed all the way to the long wall corners (but flat against the wall, not cornered). If you don't have a use for them except at holidays, they would be easy to store if you made them with tilt-up tops. You could even make them with a latch on the underside so they could clip together and be a round table! Or if you don't like demilune, you could do the same with two narrow tables: seperate to serve holiday desserts, center back together for every day use.

Sorry if this is of no use lol

I like the idea of cabinets on each side. And I think you could execute it nicely too, if your other work is any testimony.

This may be something you might like to do? I just happened across it and thought of your dilemma.

http://www.shanty-2-chic.com/2014/04/diy-console-table.html

I think if you do something narrow on the long wall, you'll be fine with two cabinets on either side of the window. If you keep it short enough, it shouldn't add too much bulk to the space.