Scenario 1: No idea, Fiamma without holy right doesn't make sense. I mean if we go with index stats for that this is likely city level vs island level durability. Well, index has some good hax, but nothing that would give her huge advantages vs much superior bullet spam.
Scenario 2: 2 possibilities to start with.
Possibility 1: Reimu decides to first shoot a bullet or something and then go fanatasy nature. In that case she dies.
Possibility 2: Reimu decides to first go fantasy nature and then begin to shoot stuff.
Now normal combat doesn't work here and reimu also has the luck of not using magic.
Has index something that would work though? Well, one thing that could work is sheol fear.
As it targets religious people specifically it works well on reimu and is not magic. Why could that work against fantasy nature? Well, I suppose we would argue that reimu is not blind when using it and likely also not deaf, so that she can hear it. Since it doesn't use a supernatural method (no mana involved) one could compare it to the ability of
elva in that it basically causes a magic breakdown by giving the opponent certain informations instead of really using a mind attack.
The other argument that I want to bring up is that a long fight will not be in reimus favour. Index has an incredible analysis and developement ability. The best example of that is her replicating gugnir, a universe level weapon, later on in the novels and that the Index Librorum Prohibitorum is hyped to be able to raise a human to the level of
a magic god. Using special anti-Shinto spells or other possible things I would believe that the Index Librorum Prohibitorum can be used to put together an effective fighting method even against fantasy nature. (Though that of course involves speculation)
So my vote: Scenario 1 Reimu, Scenario 2 Fiamma.