First is what is the point of making a high mesh and low mesh. To add to what Wes said, you don't even need a high res mesh, just "a mesh", so depending on what you are doing, don't think you need to say, make a high res version of a low poly asset. As I use SP simply for texturing my models I make in Blender, I just export the mesh and leave it at that.
What reason would there be for reupdated the importing the meshes later on as mention in the tutorial video?For me, I do that A) when if make a mistake and export something only to discover an issue later (Doh!) Or B) When I decide I want to add geometry to the model, or change the UV layout for some reason, or both. A common thing is finding you've not paid attention to overlapping UVs or some such. SP is awesome here, you can just rebake the maps, leaving all the layers alone. The only issue you have is if you have any "painted" layers, as these may need re-doing of you've latered your UV layout.
Do you need to use id if you don’t have a high meshYou don't need to, but by the same token, I recommend it as a way of controlling textures. I have a video where I use Blender and SP together, to keep the number of actual materials down, (which become texture sets within SP), as I know I am texturing in SP, I create an ID mask, so I bundle geometry together, with an Id mask, (in Blender, this is built from the vertex colour map). In SP, I then use this to isolate parts of the textureset with masks to create different textures within a single textureset.
Again though, SP means you can do it this way, or just create a mask based on UV islands or polygons to isolate things, I personally just find an ID mask easier to work with. Though I may incorporate UV poly masks as sub-sets of an ID mask.
Here's a link to part 1 of the tutorial of Blender2 substance, it has 3 parts in total:
https://www.youtube.com/watch?v=-yInmaj1-tw