From what I am told, my 8100 unit is horribly inferior to a basic Garmin when it comes to custom routes.  I'm told there is no method to make a custom route on google maps, export to .kml or any other format, and import to the 8100 unit.  If there is a way, please correct me.

So, what I'm left to wonder, is it possible to put in a destination on the unit, get to the route overview, and then manually change the routing street by street if desired, similar to dragging the route around to custom on google maps? 

Look in to your map settings tons are in there.

Look in to your map settings tons are in there.

What in map settings is going to allow me to do what I asked, changing a route at a street by street level?  Aren't all those settings about the unit doing things like avoiding highways, etc?  That doesn't really help me make a custom route.

Oh, I misunderstood your question. On a street by Street level not sure that can be done. I have to play a round and see what I find.

I think all you can do is set the A route, then change the way points , but not how it gets there

You can import a .kml file through naviextras toolbox.

Go to https://www.naviextras.com/shop/portal and click on "DOWNLOAD PC TOOL".

You have to go through the same process between your computer and head unit as when you do a backup or update.  The only difference is you load the .kml file into the toolbox extras page in the process.

As to whether the .kml file imports as a route or just waypoints that establish the route, I do not know.