Hello - I plan on doing a complete hardwood install of my 2nd floor (approx. - 650 sq ft) - I plan on starting in my master bedroom, since it has the longest run of straight wall and will come out 1/4" from the wall and plan to work my way down from the bedroom to the hallway and other bedrooms. My question, relates to how I should go about mating up to the hallway stairs sill plate - do I need to leave room for expansion against the stairs sill plate or can I come tight to it with no expansion? If I do this, will expansion that I account for on the opposite wall be enough, to prevent any issues - the recommendation I have so far is to use L molding to hide the expansion gap, but would not prefer this, since it will look odd and I recall seeing many hardwood installation where they install tight to the stair railing sill - how do they get away with this?

Hallway stairs sill plate? Not familiar with the term. Are you referring to the landing tread or the plate that the railing balusters and/or newel post sits on? Can you post a picture?

Correct, the plate that the baulister rails sit on. Not sure how to post a picture on this chain. The plate is about 3 feet in length.

I glue the nosing in place, or only use 8d casing nails if there will be a need for expansion. The nails will allow some movement, if there is enough pressure when the flooring expands. The expansion gap is at the wall. No need for any expansion gap at the ends of the rows of flooring.
If you are going to use a shoe to keep the balusters in place, and also keep stuff from sliding off the floor over the nosing to the area below, you can leave a gap between the flooring and the nosing that will be covered. The shoe will need to have fillets made that take up the space between the balusters and fill the shoe. The shoe beginns at the top newel post and ends at the wall with the balusters spread evenly between with a gap no more than 4 inches between the balusters.

Will the new flooring be parallel or perpendicular to the plate? What is the thickness of the plate? What is the thickness of the flooring?

The new flooring (3/4") will be parallel to the plate - the plate high is 1 inch - I was considering doing an undercut, but also have some other posts where they have a similar scenario, where they mate the new wood right up to the plate with no expansion - is this an option, will the expansion I create on the opposite way, composite for no expansion allowance at the plate?

If you rabbit the thick edging to keep the expansion gap hidden before you nail it down it would be good insurance against too much seasonal expansion. In my relatively constant atmospheric conditons of the Bay area in CA I get by without any expansion gap next to a nosing in a hallway.

The expansion gap along the perimeter of parallel floor boards is designed to handle extreme expansion of the outer few rows. If there is extreme expansion in the middle of a floor the boards will compress against each other and buckle. The expansion gap on one side of an area won't aid boards on the far side in the event of extreme expansion. A lot of folks will tell you they would butt this tight and not worry.....but extreme expansion is possible but not probable.
With the undercut you will have a nice neat look.

I believe the expansion you leave on the opposite wall will be sufficient for the job.