Hello @Michael Clarke   Thank you for your questions below: 

1. How can we make a Marker Layer showing:

• Accounts without activity in last 30 days
• Accounts without an Order in last 3 weeks

Here is an article that will assist you in creating a Marker Layer with Activity and Cross Objects.

https://help.salesforce.com/articleView?id=sf.salesforce_maps_setup_marker_layers_options.htm&type=5

2. Can we create marker layers from reports?

Unfortunately, you can not create a Marker Layer from a report at this time.  However the creation of the Marker Layer is similar to the creation of report.