NFL Rules Question - Penalty Enforcement

[FormerUmp]

https://youtu.be/5GEtlF6Zftg?t=3316

A few hypotheticals regarding penalties with a change of possession and no time remaining.

The only foul called was prior to the possession change. If Harrison had been tackled short of the end zone, would the Steelers have gotten an untimed down?

If the Steelers had been called for an illegal block on the runback, what would the result have been, given that the foul on each team would have been separated by a change of possession?

Obviously this play was quite some time ago, but it raises some questions that aren't obvious to a non-official. I'm primarily curious about the NFL, but I'd also be interested in how NCAA or NFHS would deal with it if anyone is so inclined.