Multiple foul??
 

4th and 20 for A and they elect to go for first down. A throws pass downfield, B gets flagged for defensive pass interference and B intercepts. There is also a flag for a hit by B on a defenseless player after the interception. A accepted the pass interference penalty and declined the hit on the defenseless player to give the 4th and 5. Was this done correct?

If I'm following you, on a 4th & 20, B fouled A (prior to a COP) by Pass Interference. then after the play was over committed a dead ball PF. A accepted the PI penalty (15 yds from previous spot) and repeat of 4th down. The dead ball foul does not combine with the PI live ball foul to create a multiple foul situation, so A had the option to either accept an additional 15 yards from the succeeding spot (4th & 5), giving them a 1st & 10, 15 yards in advance of that spot, or declining the DB foul and replaying 4th down, with 5 to go.

Only A can explain why they declined the DB foul.

The hit on the defenseless player occurred after the interception but while the ball was still live during B's return of the interception.

Only one live ball foul can be accepted and they don't want to decline the DPI and accept the PF because then Team B would keep the ball. This was administered properly.

Sorry for the misunderstanding, as advised by NFHS: 10-2-4; "When 2 or more live-ball fouls (other than non-player or unsportsmanlike conduct) are committed during the same down by the same team (multiple fouls) only one penalty may be enforced.The offended Captain may choose which one shall be administered, or the Captain may decline all penalties."

These options, most often, require slower, more detailed explanations.