What I would like to know is whether any other US laws, or any current US policies, apply when a US citizen becomes a member of a foreign government and takes an oath of allegiance to a foreign state.
Yes, there is an additional policy to consider. There is an administrative presumption that the individual did not intend to lose US nationality if the position with the foreign government is a "non-policy-level" position or the oath is a "routine" oath of allegiance. This presumption is established by regulation at 22 CFR 50.40.
The US Foreign Affairs Manual elaborates on the circumstances in which the presumption applies in its section on developing a loss of nationality case. There is also a section specifically about loss of nationality and taking up a position in a foreign government.
These make it clear that if the position is a policy-level position, no presumption applies. Similarly, if the position is a non-policy-level position but the individual states an intention to relinquish US citizenship, the presumption is also inapplicable.
If the presumption is inapplicable, then the consular officer must develop the case by examining all of the available evidence.