After a snapshot has been taken, there are no more transactions, only the account balances. Which tips are referenced as parent by the first (new) transactions? Is it possible to use one transaction as a parent after a snapshot while the other parent comes from a subtangle without snapshots?
Which tips are referenced as parent by the first (new) transactions?
An answer for this question can be found here: Clarify inner workings of the snapshot
The genesis transaction (with hash: 999⋯9) is directly approved by the Coordinator. The first transaction after the genesis simply approves the genesis transaction twice. In other words, it has the same Trunk and Branch which is the genesis transaction. Tip selection algorithm is a random walk, so it is probable that the Trunk and Branch are the same. On a healthy Tangle the probability of this happening is very low, and vice versa.
Is it possible to use one transaction as a parent after a snapshot while the other parent comes from a subtangle without snapshots?
From the Tangle whitepaper (page 2):
If a node finds that a transaction is in conflict with the tangle history, the node will not approve the conflicting transaction in either a direct or indirect manner... If a node issues a new transaction that approves conflicting transactions, then it risks that other nodes will not approve its new transaction, which will fall into oblivion.
All pre-snapshot transactions are simply not a part of the Tangle history anymore. Therefore, it is possible for a node to issue a transaction which approves a pre-snapshot transaction, but such a transaction would not be approved by other nodes.