I grew up with tomatoes being spaced 24″-36″ apart. However, in our very small plot, they are planted closer together, about 15″, with tall cages. I have them in rows 15″ apart as well.
Some of the indeterminates have just-starting-to-yellow lower leaves. Is this likely caused because of being too close together? Is there a way to save the plants?
Lower-level leaves that turn yellow-to-brown on tomato plants isn't always a sign of disease in the plant or a failure that requires "saving." You've got plenty of space between them so if the rest of the plant is healthy and growing, the lower leaves simply may not be getting as much sunshine as the rest of the plant. That's usually not the case until the plant is pretty mature, though, so take a look at other possible reasons here: