Can the confinement required by CCP 42.12(13) to qualify for community supervision in DWI cases be waived by the court? If not, can the court give defendant credit for time served at the time of arrest to satisfy the 42.12 requirements?

Don't think so - see Martinez v. State 427 SW3 496. HOWEVER, note that in Class A DWI cases, where the ignition interlock device is ordered, that the requirement of 5 days of confinement (as well as the minimum confinement periods in 42A.401 (3), (4) & (5) do NOT include "continuous" as in 42A.401 (1) "not less than 72 hours of CONTINUOUS confinement..." Current CCP references are to Chapter 42A since Chapter 42 was repealed. *And thanks to O'Connor's for the case reference

As a practical matter, if the judge and both sides agree to waive it or give credit for prior time served, then who would mandate it to happen? I've seen it done before, as long as everyone is in agreement.

Seems a slippery slope when judges intentionally do not follow the law.