Q1 Is it possible or ill advised to have two POPs one at either end of a long chain of DNs to be precise there would be 21 nodes between them ?
Q2 If the above is ok , is it further possible to have third POP that joins as a T-intersection near the middle ?
Sounds like the perfect deployment, the chain will split in to two zones, one for each POP. If one POP was to do down all the DNs would use the remaining POP.
Whilst in the user guide we recommend 15 hops as a max, this is not a hard and fast rule. We have networks with 30+ hops and the latency is still <10ms.
In the future if you wanted more capacity you can convert any of the DNs to a POP and the DPA would reassign the prefixes and divide it in to 3 zones automatically. There is an outage when you do that but it should be a minute or so (using the latest software 1.2.2.1).