...about the settings when rejoining replays i'm not quite sure, either the initial one is used or the assigned one, no idea. you can get single cars from a series to run, or create your own custom series using the build-in mod creation tool. it's much easiert han i initially thought, and there are a couple of tutorials on youtube. its located in \rFactor2\Support\Tools and is called "mas2.exe" use the box symbol on top (middle) "create the package file" then "create new mod package" check "customize" and edit the "default.rfm" file in case you like more than 30 cars on track: line "Max Opponents = 103" is maximum. don't forget to save. Next>>> choose track (s) Next>>> choose series Next>>> do nothing Done In this next window you can then choose single cars from a series. select a series in the "Vehicles" window in the "Teams" window you can now select or unselect single cars (e.g. by car number) by using control+left mouse. you may need to scroll all the way down initially in the "Teasms" window to have all cars displayed. then click "package", install the mod and have fun!!! you may edit your mod later by using the "edit existing mod package" button in the box menu from the beginning. e.g. if a new version of a car or track is out. (in-game you hace to select your mod instead of "all cars all tracks, and here you are!)
Maybe the OP did, but I found this thread looking for exactly the same thing, and what I linked to seems like a good solution (at least for my purposes) Just cus a thread is old, it doesn’t necessarily make the information in it irrelevant.
I joined in 2016, didn't even know this thread existed until you bumped it And considering that back in 2013 nobody suggested it, my guess is that feature maybe got added later.