You can check flatness by taking a pair of tiles and placing them face to face or back to back. If there is any gapping around the edges or in the center when you look at them from the side, then they are not perfectly flat. That doesn't mean that they can't be used. It just means that they will look better if they are not laid in an overlapping pattern where you get lippage, and it is important, if they are bowed, that it is consistent from tile to tile. I did a small bathroom in 18" square tiles that were perfectly flat and I still chose to align the tiles in a standard, straight set pattern. Lippage

When I suggested running it the long way I was considering the best look for the bathroom beyond too. I like a long edge to run into the room. It will make less cuts by the tub too so the edge is tidier. I don't know how wide the hallway is but to me the layout looks very chopped up since there's only one full tile in each course. With the 1/3 offset that an amateur should probably use it will be even choppier. It will be one large format tile next to a sliver of the other. The pic Bill posted is beautiful, but it looks like the hall pattern follows the pattern that was chosen for a larger space. That's why I like the stacked pattern here, running the length of the hall and then continuing into the bathroom.

Plenty of rugs won't save you. I was giving the lab a bath. He bolted out the bathroom door after the bath and I rushed to close the door because it was drafty. He scrambled the rugs on the way out, I slipped and fell into the door jamb and broke my wrist. Would not use the tiles that are linked on a floor. They have R9 Slip Resistance rating which is the least slip resistant rating.

I‘lol comment on the “quality” aspect a little. My previous house had tile throughout the main living areas and it was laid on the diagonal. Initially when we looked at (&bought) the house we too thought it was different/special/unique/whatever. Friends commented on it looking nice. However, it just really sucked. Rugs and furniture placement always looked bad bc it was hard to get them lined up with the angle or exactly perpendicular to it. It quickly became a PIA. It never just blended away as a floor. I know yours is a bathroom but I would still rethink it. It could easily end up looking like you just couldn’t make a straight line. Maybe use a smaller rectangle tile for herringbone or another interesting layout or find some interesting tile and do a stacked or offset pattern. I think you’d find it nicer and less “unique”. Sometimes unique is the equivalent of bless your heart.