I started a woodland path last year and use fallen branches to edge the path (3 or 4 inches) and everything else gets mulched and spread on the pathway. My paths are curved as well. The mulch is very light in color and takes a while to darken (1/2 year?) I really don't know yet how long it will last.

I think it will look o.k.
I have used fallen trees, branches, logs, cut trunk sections and anything else that is handy to line paths and it all looked fine. I have "paved" paths with woodchips many times and that worked well. Fallen trees will probably last as your path edges for at least a few years. A lot of factors go into how fast they rot including site conditions and the kind of tree but bigger trees tend to take longer to fully decay just because there is more stuff to break down.

I got the idea to line my woodland garden beds with logs from the wild area at the Chicago Botanic Garden. They used logs between 6" and 12" diameter. For curved sections, the logs can be cut and angled. I have done this for 12 years, and have had to replace the logs once in that time. I like the look of a woodchip path lined with logs/tree trunks. I wish I could show you a pic of my garden but I just can't seem to get the hang of it on this site.

I do not like the use of branches or trunks to line paths. I like my woods to look like a woods and lined paths do not cut it for me.