Disclaimer: I do not answer your question. 
You are correct that the Transaction Tree does not include explicit mention of the archiving of the contract.
That is consistent with what we see in the Daml Studio Transaction View.
That is also consistent with the fact that Archive is an auto-generated choice on the contract. And, to be honest, you are not exercising that choice, like this example:
There have been related discussions on these two posts:
- Explicit archives on the TransactionTree stream.
- Why isn't there an archive node in the transaction graph on a (consuming) choice - #2 by cocreature
So, how can I get the archived contract that happened during the exercise of
DoSomething?
