Hi! Bear in mind that as a developer of Elastic APM, my answers might be a little bit biased 
Maybe someone from the community will chime in too.
I would say the only benefit of using Jaeger in this case is if you're already using Jaeger -- that means you don't have to change your instrumentation. When you're using the Elastic APM UI, the Elastic APM agents will always provide the best experience, seeing as they are designed to be used together from the start.
From https://www.elastic.co/guide/en/apm/server/current/jaeger-reference.html#jaeger-caveats
Because Jaeger has its own trace context header, and does not currently support W3C trace context headers, it is not possible to mix and match the use of Elastic’s APM Agents and Jaeger’s Clients.
Since that statement was written, support for W3C TraceContext was added for the Jaeger Java client; other languages do not support W3C TraceContext. If your Jaeger-instrumented services are written in Java then it should be possible to mix Jaeger and Elastic APM agents across your services.
Yes.