1. Would R12.1.5 need separate license? Or, R12.1.3 license would cover R12.1.5 as this looks like an incremental upgrade.Same license (assuming you already have HRMS license).
2. Not finding good documentation on R12.1.5 on metalink. Is there a RVP that can be shared if any of you have it with you.If you are referring to HRMS Release Update Pack 5 for Release 12.1 then you should have the docs referenced above.
3. This probably should have been my first question. Is R12.1.5 a public release? I ask this question as I do not see good information available on upgrade road map to R12.1.5.The latest 12.1.x release is 12.1.3 and Oracle has no plans to release any higher 12.1.x releases -- The next release will be 12.2
4. Can these features in R12.1.5 be applied as one-off patches on R12.1.3?Again, if you are referring to HRMS Release Update Pack 5 for Release 12.1 then the answer is "Yes".