Hello,
I am about to start my first flooring job, laying 3/4” x 2-1/4” prefinished red oak in 3 rooms and an L shaped hallway as pictured below in brown. The subfloor is 3/4" plywood, I'm in New England, and this is Bellawood. The total square footage is about 650 sq/ft. The joists are horizontal in the below image orientation.

I have undercut the stone fireplace, and do not plan to frame it. Walls A, F, G, I, and L have hot water baseboard radiators on the majority of them.
My initial thoughts are to snap a line along wall C, and start from there in both directions with a spline. My concerns:

Should measure out from wall E or A to snap the starting line down wall C?

I’m a little nervous about the sides of the fireplace because the undercut follows the stone and they will need to be scribed. Fitting a piece in there the right width as I go could be tricky?

I had a thought to stagger a right angle direction change (similar a herringbone join) going from the living room into the hallway (from corner of wall C to corner of wall E) to effectively rotate the board direction 90 degrees. Then do the same in front of BR1. Would this be a good or bad idea? I assume I would have to router some grooves into the cut board ends?

Advice on any of this is very valuable to me. Thank you in advance!

I would just start off wall a and go with it

I'd say start off wall A as it simplifies things. Once you reach the fireplace it's just a matter of measuring how far away you are and how much space there is to slide under. Wait, maybe you couldn't undercut? Irregular stone?

I would avoid the directional changes especially the one you're thinking...outside corner to corner? Odd location. Directional changes are not very common anyway. This way you have the flooring running lengthwise coming in the foyer (more preferred)

As Jeff says...Get her started. Reach the stone and we can go from there.

He said the "B" word.

Starting from wall A would be the easiest way to go about things. Generally, moving from the outside wall of a room makes it easier to make sure that your flooring will line up with the rest.