Is there any way to apply a right side master in a spread to a left side page?
If not, is there a way to split all master page spreads into single master pages, so they can be applied to both right and left side pages?
A new 'spreads' document always starts with a double master page and no, there is no way to apply a right side from a spreads master to a left side page.
However, you can create a new single page master via the 'Pages' panel, which you can then apply selectively to left or right-sided pages. Just make sure you type '1' in the 'Number of pages' field. At which point you can also remove the initial 'spread' master and add as many single page masters as you need. And move any original artwork from the double master to the single masters.
To apply a master to a page just drag it over with your mouse or use the 'Apply Master to Pages' via the 'Pages' panel.