Selecting a node by double clicking its tab is a good idea.
I'm not quite sure what you're asking in your second question (not sure what you mean by "secondary dynamics"), but Birth Flow and Update Flow should be pretty fast, even up to millions of particles. It directly reads particles from one flow to another, without unnecessary calculations, so it will be efficient. However in your setup I'm not quite sure why you're wanting to import particles from one flow to another, instead of keeping everything in the same flow. Maybe you can elaborate on what you're trying to achieve.
I'm not quite sure what you're asking in your second question (not sure what you mean by "secondary dynamics"), but Birth Flow and Update Flow should be pretty fast, even up to millions of particles. It directly reads particles from one flow to another, without unnecessary calculations, so it will be efficient. However in your setup I'm not quite sure why you're wanting to import particles from one flow to another, instead of keeping everything in the same flow. Maybe you can elaborate on what you're trying to achieve.