Woods going to expand and contract. But it wont close a 1/2 gap. Ripping a board down will be just fine. When the planks gap out that much first thing i would do is clean the gap as much as possible. Im sure the inside edges are caked with gunk. Clean that well, glue in your wedge pieces, and finish away. Are you refinishing the entire floor? Or just plan on finishing the wedge pieces? Its hard to tell but my guess is the exsisting finish looks like a varnish. With that in mind you cant put your typical polyurethane over varnish or shellac. It doesnt adhere properly and will just flake off. So you can find varnish still and i would use that if youre not up to the task to sand down to bare wood. If you watch This Old House they did a show where they put pieces of rope between the planks. Looked ok. Not a fan personally but its another idea. Being the house is as old as it is you cant really go wrong looks wise. Good luck!

OK, my take on it, is to get the interior up to the highest rH it will see in the humid season(NOW), for a week or longer. Then fill the gaps. This allows the wood to swell to its largest dimensions, and won't squeeze the filler out, as it will, filling during the winter-heating-dryingout, months and come the humid months and the floor swells. During the winter, the floor will shrink, unless you have a humidifier. You will see gaps appear.

Well, being concerned about the step down safety, and also wanting a nice even floor (track for grandkids) are valid points too. Something else to consider, maybe its time to take the money you save on floor-raising, and think about that next home for you. What will the best layout be, for you, for the next decade, or even two? More main floor living? Fewer stairs? Kitchen drawers instead of cabinets? Curb-less shower? Big back yard for gardening? Or no outdoor maintenance at all? When will be the right time to move to that house? Or do you plan to stay where you are until you need assisted living? In which case, spend whatever you can afford to make the home work for you now. Just know that you likely won't get that money back.

We lived in Florida. Wood moves with the seasons, so glue would not allow the movement. Since we laid the floors ourselves in 1997, am working from memory here. We had plywood subfloor down then stacked the flooring in the air conditioned house for several weeks. We face nailed only. I really wanted an antique look and we used cut nails. All those are hammered in by hand. After all the flooring was installed, I mixed two colors of MinWax stains, Puritan Pine and one other ( memory fails me) in a one gallon paint can (new empty cans are sold by paint stores and Lowe’s) so my color would be consistent. No one else can be in the house for over a week while the staining and oiling are being done. Authentic Pine Floors gave me a formula for the finish I mixed in another unused gallon paint can, but I think I would use straight tung oil now. Tung oil must have five or six applications with sufficient drying time between coats. This cannot be rushed. We used tung oil on the cherry floors we laid two years ago in the kitchen of our current house and love it. Our Florida house had carpet upstairs for noise control, so I estimate we had over 2000 square feet of pine floors. My sister used the same material for her house in the Midwest 16 years ago and the floors are still beautiful. My daughter has about 3000 square feet of the same flooring, finished the same way. We all have dogs and active lives. Some people age their floors by dropping chains onto the surface, spreading sharp gravel on the floor and walking around with work boots to scratch the floors, or you can (with extreme caution) roll burning logs onto the hearth area to put a few scorch marks. Then stain and oil the floors. We did none of that at any of our three houses, preferring to let the floors show our history. All wood moves with humidity. It is not much, though. It certainly is not huge gaps.