If your understanding of the regulation was true, then if you divert to your alternate making it your destination and it required another alternate, many would be in violation of the fuel requirements. Also, you do a proper plan and carry sufficient fuel as required by 91.167, but the head winds encountered enroute are much stronger than forecast, you could easily eat into your reserves and no longer meet the planning requirement. So you land with 30 minutes fuel remaining at your alternate, you have not violated any regulation.
Perhaps I was cavalier in my use of the term destination. I guess it depends on the interpretation of destination vis a vis the first airport of intended landing. I have not yet found any published interpretations for those particular phrases relative to each other. I acknowledge up front that what follows is my interpretation, and would welcome references to any other published interpretations.
In my mind, there are two destinations described in the IFR flight fuel requirements: the
first airport of intended landing and the
alternate. Each destination has its own fuel requirement, and the pilot may elect to change one or both of these destinations while in flight.
In your first scenario, I would argue that when you divert to your alternate, you have not changed your first airport of intended landing. You are simply proceeding to your alternate early. Presumably you would land there with even more fuel than you would have if you attempted an approach at your first airport of intended landing. As long as you land there with at least 45 minutes of fuel, you have met the requirement.
In your second scenario, the FAA agrees with you that if you eat into your reserves, you have not violated any regulation. See
INFO 08004, which includes the following statement:
The act of using a portion of the reserve fuel assigned to a flight is not, in its self a cause to declare a minimum fuel state with the controlling agency. Regulations require reserve fuel to enable aircraft to maneuver, due to unforeseen circumstances. Many aircraft safely arrive at their destination having used a portion of the fuel designated as reserve.
In my earlier response, I was talking about keeping the fuel requirements in mind as you
reconsider and perhaps change your plan.
Consider a scenario where you shoot an approach to your first airport of intended landing, go missed and then elect to attempt an approach to a nearby airport that's not your alternate (because your alternate is 50 miles away). Can you do this? Of course. It doesn't have to be in your preflight planning, per se. But you are now, in effect, designating either a new first airport of intended landing or a new alternate. If you are designating a new first airport of intended landing, you must have enough fuel to complete the flight there with enough fuel to continue to your alternate with 45 minutes of reserve. If you are making it your new alternate, then the forecast and actual weather at that airport must meet the alternate airport weather minimums, and you should land there with at least 45 minutes of reserve (barring unforeseen circumstances).
Consider another scenario. After a long VFR cross-county, your destination airport gets covered over with a marine layer of low overcast clouds. It's an easy instrument approach to your home airport so you ask for a pop-up IFR clearance. Can you do this without filing a flight plan? Yes. ATC has the authority to give you a pop-up clearance without a pre-filed flight plan. But you are still subject to the fuel requirements, so you can only ask for that IFR clearance if you have enough fuel to complete the flight to that destination (now the first airport of intended landing) with enough fuel to proceed to an alternate and land with a 45 minute reserve. This implies that before asking for that clearance, you need to consider what your alternate airport is going to be and whether you have enough fuel to shoot that first approach legally.
