Inreach Hangup/Debug on Route Creation

I'm not sure I understand the problem you describe in (a). I am not seeing this behavior with Chrome, at least not on Win10. On the other hand, the steps required to "move" the map are a little bit counter-intuitive, at least to me. I'm going to describe what works for me. If that's different from your experience, please explain in detail what you're doing differently.

Center the map on the beginning on the route. Start your route and work your way to the edge of the map in whichever direction is appropriate, just as you normally do. When you create the last route point near the map edge, do not release the mouse after you click to create the point. Instead, drag with the mouse away from the edge of the map. Think of this as "dragging more of the map into view". Now release the mouse, which actually creates the point that was previously on the edge. Continue creating your route until you reach the edge again. Repeat as needed.

It is possible to change (or extend) an existing route. However, the only technique of which I am aware is extremely cumbersome, especially for extending a route. You are correct to assume that "getting it right the first time" is the best idea.

The best way I have found (and there may be better ones) for extending a route is as follows. Click anywhere on an existing route to start the process. This shows a pop-up "bubble" which includes a pencil icon. Click the pencil to start editing. You will note that the route line now shows alternating "solid white" and "semi-transparent grey" squares. The solid white squares are actual inflection points on the route. The grey squares are so-called handles. Clicking a handle will turn it into an inflection point and will add two new handles, one on each side. To extend a route, the easiest thing I have found is to start by clicking and dragging the last handle. Drag it almost all the way to the final inflection point. Release the mouse. Now, drag the final inflection point to extend the route. Repeat the process until you have what you want. Don't forget to click the green check mark icon in the "bubble" to save your changes.