Hi Mike,
The last error 'getValue of null' is due to incorrect schema name of regarding field. It is regardingobjectid not '_regardingobjectid_value'.
However this is not you need to do. You need to retrieve the lead lookup value from the case entity using web api (refer the previously shared blog). Once you get the details of the leade (i.e. name, id), you set the to field as follows-
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var partlist = new Array();
partlist[0] = new Object();
partlist[0].id = "ff4070a3-d5eb-e911-a812-000d3a7957a4";
partlist[0].name = "Susane";
partlist[0].entityType = "lead";
formContext.getAttribute("to").setValue(partlist);
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Hope this helps.,