Yes, the floor joists run East to West (left to right). I would prefer to make it simple as you suggest and just continue the horizontal direction, but I keep reading that it is not a good idea to run hardwood floor parallel to joists.

I don't have a picture (I can grab get one if you like), but I did the same thing in my current project - new wood floor in the kitchen going right into existing in the dining room. The floor installer feathered some of the new boards into the dining room floor so it looked continuous, then I had all the existing floors sanded and refinished and stained the old+new 1 color. The result is a transition that looks completely continuous and uniform. Of course I had to refinish all my existing floors to do it. If you don't want to do that the only thing I can imagine working is a perpendicular strip to edge the new and old. It'd be impossible to match the old stain with new perfectly so you'd need something to separate the two. I have a spot in my house where the boards are mismatched so whomever installed it back in the day put a strip. I attached a picture of that.

We have hardwood on our main floor and LVP on our stairs down to the basement as well as on the basement floor (over cement). When the LVP was first installed on the stairs (it's been a little over a year now), my husband thought the "lip" part of the stair treads (the bull-nosed edge piece) was not installed perfectly flush and secure on several (I think 4) of the treads, so I wonder if that is not uncommon and is what your flooring contractor is worried about. I didn't even notice it, but my husband is a perfectionist and it bothered him. Rather than complain and have the installers come back to address the issue, my husband fixed the issue himself. I think he used tiny little nails (with no heads) to better secure the edge pieces to the adjoining LVP pieces, and I used dots of acrylic paint from my art supplies to cover the bitty nail holes. We have never had a problem with them since and we are up and down the stairs many times daily so they get heavy use. Just wanted to mention our experience in case that makes any difference for your decision. I don't regret using it on our stairs. We picked an LVP that coordinates pretty well with our upstairs hardwood in color, just a little more rustic which fits our basement vibe. We didn't want to introduce yet another type of flooring just for the stairs.

Well the kitchen is almost done. The floor is my favourite thing! Absolutely love them! It is porcelain wood plank layed herringbone butting up against the hardwood floor. The hardwood and porcelain floors do not look the same but they do have the same tone. I think they look great together. It was hard to get a good pic because the lighting is different in the kitchen vs the hall.