Thankfully, it turned out to be an easy job and no further complications occurred. David applied a 3rd coat of paint on the top-half of the accent wall (basically to even out the sheen vs sheenless parts). It worked like a charm. When applied, the paint looked lighter in color. But once the paint dried (which only took 10 minutes since he applied a very thin layer), it looked identical to its neighbors. Phew!! Hooray! What a relief! :)
I had mentally prepared myself to accept the worst case scenario--that we would have to repaint all walls if somehow the spot-coating would not blend seamlessly into the original coating. (One loses naiveté about having things go your way when too many things has gone wrong.)
I guess this is what having experience means--you are mentally and emotionally prepared for setbacks. However, idealism vs experience is a cruel trade, if you ask me. Too much idealism and you forget to put in honest effort and plan with enough buffer. Too much experience and you become a cynic whose motto is: "That won't work.". One shouldn't reside on either extreme too long if at all possible.
So now that the painting is done, I plan to retry my hand at stenciling maybe tomorrow evening. Wish me luck!