Does one now (or ever in the past) have to listen to the instructions of an angel? If so, would that include listening if the angel told you to violate a Torah commandment (either temporarily or permanently)?
The Rambam writes, both in Hil. Yesodei HaTorah (7:6) and in Moreh Nevuchim (2:34) that, with the exception of Moses, all prophets received their prophecy through angels. As we are obligated to obey a prophet, it would seem that the fact that the prophecy was conveyed through an angel is not a problem. This would include a temporary command to violate a Biblical commandment (with the exception of idolatry). (Hil. Yesodei HaTorah 9:3-5)
However, not every apparent revelation qualifies as full fledged prophecy. The Rambam discusses the rules for determining when a person is a full-fledged prophet in Hil. Yesodei HaTorah (chapters 7-10). If the prophecy is not a full-fledged prophecy, then one certainly would not be obligated - in the full sense - to obey, especially if the supposed command was in violation of halacha (in which case one would be forbidden from obeying the command).
Whether such a revelation could be used to help resolve a halachic question, or whether - in the absence of any halachic issues - one would be best advised to follow such instructions, is a far more complex question. In this regard I would recommend reading R' Reuven Margolios' introduction to his edition of שו"ת מן השמים (printed by Mossad HaRav Kook), where he discusses the various lower forms of ruach hakodesh that have existed in the generation since the end of prophecy, and how much authority they are given in halachic matters.