Call Sv_ShutdownServer rather than Sv_Shutdown on ERR_NONE and ERR_DROP. This way the server can be restarted (e.g. 'map foo').
Rename netchan to net_chan.
Split long client datagrams into multiple packets and transmit serially. This essentially removes the 'client overflowed' condition, provided no single server command exceeds the 1400 byte limit. SVC_CMD_FRAME is the only likely offender here. Spawn 100 entities in a single frame and you'll likely still overflow.

Also re-introduce the 3-frame pad at the beginning of the game to allow entities to settle to the floor and whatnot. This way, they produce more useful baselines and smaller deltas on-connect.