fromLocal is only true if, when you do sendToNearest(), the local node is the nearest (that is, the sender was the local node). this is how you implement recursive routing, since there's no other way to examine the internal routing table.
estimatedMemberCount is an estimate of the total size of the group. it is computed based on neighbor density. you may not be directly connected to (that is, neighbors with) every member of the group. in general, you should have O(log N) direct neighbor connections in a group of size N. the actual number will be approximately to 2*log N + 13. groups will be fully meshed below about 14 members, and are likely to be fully meshed up to about 18 or so members.
i go into more detail about this in my session from MAX 2011: