In a multi-replica system, where, say, we cannot tolerate any failures or lags, is 2PC used in practice to achieve consensus? Or are there other methods for achieving such strict consensus?
There are other means, the most common being Calvin like protocols where every replica deterministically processes some log, and so only one round of distributed calls is necessary.
2PC (or atomic commitment more generally) is needed for sharded/partitioned systems with different data on each node. In these systems, each node gets a vote on whether a transaction should be allowed to commit. Replication, making multiple copies of the same data, doesn't need 2PC. Instead, algorithms like Paxos, Raft, or chain replication are used.