My theory is that Regret was giving his speech in an archaic dialect that the UNSC had little experience with, so it wasn't in the Master Chief's translator.

Mercy, on the other hand, was talking to the Chief in a common Covenant tongue, because he wanted to be understood by the Demon.

: 2. Was 2401 Penitent Tangent compromised, or did he say all of the perfect
: lines of his own volition?

Probably compromised.

: 3. The Halos are put on standby activation, which 343GS says means the Halos
: can now be fired from the Ark. So otherwise, they can't be fired from
: the Ark? That seems like terrible contingency planning. (Also, in
: Silentium, the IsoDidact does fire them that way.)

I'm going with Loftus on this one.