Will replication commands build up if the distribution agent is stopped?
Yes, the transactional replication data will build up while waiting for distribution to start again.  Of course, if not tracked it is possible that a long delayed set of transactional replication data can fill the log space that you have available, thus causing you problems.  

Replication is hostage to the resources available, including the log space since the replication data is read from the transaction log.

For what it is worth, we once kept a replication open for a couple of days until a network problem could be resolved.  (Plenty of log space at that time.)

If the replication process fails (for any reason) that would cause loss of data then you need to redo the steps to establish the replication landscape.