If the flooring manufacturer's instructions call for expansion gaps then it would NOT be ok to butt the flooring tight up against the moldings. This would essentially have the flooring trapped between the moldings and if the flooring were to expand the boards would get wider and push against the moldings.

I understand that. I guess the thing that I get confused about is the fact that on a nailed down floor each individual board is fastened to the subfloor so that it cannot move. So if each board expands, wouldn't it essentially be trapped between the adjoining boards, which are also expanding? You mentioned in an earlier post that the stair nose molding could be acting like just another piece of flooring when it is at the top of the stairs. I guess I just don't understand why the moldings on each side of the OP's catwalk are any different. Is it the way that they are fastened to the subfloor?

I understand that. I guess the thing that I get confused about is the fact that on a nailed down floor each individual board is fastened to the subfloor so that it cannot move. So if each board expands, wouldn't it essentially be trapped between the adjoining boards, which are also expanding? You mentioned in an earlier post that the stair nose molding could be acting like just another piece of flooring when it is at the top of the stairs. I guess I just don't understand why the moldings on each side of the OP's catwalk are any different. Is it the way that they are fastened to the subfloor?

Ideally flooring should be acclimated to the midrange of the annual average moisture content for the installation location. After that the relative humidity should be kept within a defined range to prevent the floor boards from swelling or shrinking enough to cause a problem. If there is excessive swelling due to a flood or excessive humidity (seacoast type environment) the outer rows would expand into the expansion gap and boards at the center of the floor would push against each other and buckle upwards. If there were no expansion gap the flooring would push against whatever it was butted to and possibly damage it. It could potentially push walls or pipes. In this case it could crush or loosen the moldings that the railings are attached to.....this is abnormal (excessive) vs normal (minimal) expansion.

Look at it this way....the stair nosing scenario affects just the floor.....the moldings scenario affects structural components.

Any update on how this turned out?

I had a similar issue before - 3/4inch sill plate, 3/8 inch flooring.
I undercut and slid the flooring underneath. It was a 4foot long section so doable.

If I had 3/4inch sill plate and 1/2 inch flooring, I really have a hard time thinking that
the lip remaining after the undercut wouldn't eventually crack and break off.

A 1/4" lip would be fine if it were supported by the flooring underneath it. 1/4" is the typical edge thickness for T moldings and reducers.

Ok.... undercutting with a jamb saw it is.

I did that to establish the top of the cut, but the found it to be a real pain to clear out
the material below it to actually make space to slide the floor in. (The saw after all is not 1/2inch thick.)

Suggestions as to how best to do that?

Can the blade be lowered to do some of the clear out? If not, you could use an oscillating multi tool or a wood chisel. Also, you could use an angle grinder but you would literally be cutting on an angle and wouldn't be able to cut too deep.

The jamb saws I've seen/used do allow for the blade to be adjusted, but when I did
do that I found it fairly awkward and inefficient to be doing that. Was just wondering if anyone knew any cute trick for it.