If that tile has already been installed on the floor without centering it to the step that was the first error. It could have been centered to the step which is a focal point and allowed to be slightly out of balance along the walls and no one would ever notice the walls.
Now the only thing to do is to rebuild the step, it's not a big deal woman or not.
Deflection is always an issue of all wood floor structures and tiling over wood is an issue that shouldn't be approached by a DIY'er. I can see from here that that tile is being installed over CDX plywood and that's a no-no. To install tile over plywood sucessfully the plywood must be
exterior grade underlayment plywood and no less. CDX contains voids and will blister easily when moisture is present.
The thickness of the plywood is great but the top layer should have never been glued to the first. It should only be nailed or screwed and not attached to the joists below, only attach it to the plywood below. The laying at a right angle is a good thing though.
Installing plywood over concrete usually isn't the best thing to do because all concrete on or below grade will repeatedly contain moisture that is naturally trying to evaporate. That evaporation process will likely work on the plywood detrimentally over time and that type of plywood won't take the moisture issues for very long. The tile installation will heave in a short time compared to how long it would have lasted if done properly due to the plywood seperating and blistering from the moisture.
That "reputable pro" has never heard of the Tile Council of America's Handbook for the Installation of Ceramic Tile obviously. And you also can't depend on getting any kind of a straight answer from the HD employees, last week they were flipping burgers at Burger King for minimum wage and this week they are tile pros.
Your only hope for the tile staying with the plywood until the moisture causes an upheival is if you are using unmodified (dryset) thinset and mixing it with a polymer additive. That's the standard your "pro's" should have told you about.
Metal tile edging is available at HD at around ten bucks for an eight foot stick, you can see some of the tile edgings at http://www.schlutersystems.com
click on "PRODUCTS".
You could install a decorative band (four inches wide, or so) around the bottom step (on the floor) using all the same color tiles (black) that you already have, this would disquise the offset step location and make things look a little more professional. It would also take the spin out of somebody that is trying to negotiate that step in a checkerboard room. A four inch wide band on the floor on three sides of the step, white risers on three sides of the step, black tread on the top of the step. I realize this isn't in keeping with the checkerboard design but I think you are dealing with an optical situation that will rapidly become a safety issue.
The metal edging would go around the black tread tile on three sides as the tile overlaps the riser tile. The two riser corners would also need metal edging down to the floor to hide the raw edge of the tile. The tiles on the front of the riser would overlap the tiles on the side of the riser.