If trolls are mere animals without any magic or spirtuality in them - How can they sense that someone is a Christian?
It could be due to a simple dislike of humans in general.
By picking up on very small traces of incense, sacramental wine and holy wafers, possibly. Although that would only work if the person had been to church recently.
Alternately, the trolls' sense of smell might be sensitive enough to detect changes in human brain chemistry, like dogs can be trained to detect oncoming seizures. They aren't detecting Christianity specifically, they're detecting monotheism! After all, they apparently had the same reaction to a Muslim...
If the Ringlefinch had rabies, why did it go down under the bridge to attack the goats from beneath? Rabid animals can't swallow water, so they tend to grow distressed around it.
It could have been at an earlier stage of infection. I know very little about diseases, but I think the hydrophobia comes at an later stage of infection.
The implication is that the jotnar was patient zero for the rabies outbreak and infected all the other trolls. So how did a solitary troll living in Arctic mountains above the snow line get infected with rabies, let alone an enormous animal that has no natural predators? Norway has officially been rabies-free since the nineteenth century. Did somebody do it deliberately with an agenda?
An infected animal such as a wolf or a bat that got in from the Russian border could have infected the jotnar, in the same way mosquitoes can spread malaria to countries where malaria is, traditionally, unknown. In fact, this is actually how Cujo started, with the titular dog being bitten by a rabid bat.
Given how the bridge troll began consuming one of the bait-goats when it was still alive and bleating, maybe the jotnar caught itself a rabid animal, swallowed it alive, and got bitten on the tongue in the process.
Considering the end of the film states that none of the teenagers were ever seen again, it's likely that Thomas never got to a hospital. Good luck with that rabies treatment.
Given that Thomas already appeared to be showing symptoms of rabies getting help would have done very little, anyway.
Just where did Hans get that bucket of Christian men's blood?
Since he's a government agent, he probably has the resources (or knows someone with the resources) to get it from a blood bank.
A blood bank would seem like an unlikely source, since they don't generally ask the religion of the donor. Someone working within the TSS could have easily given it just for this purpose, though.
Blood banks do keep records of the donors' names and personal details. It's established the TSS are government-funded, so they could look for a donor's name in census listings and work out which were Christians.
Or they got it from a blood drive held at a church.
There are probably Christian employees among the TSS, who donate blood as their way of contributing to the cause. They wouldn't be able to help out in the field without getting attacked, after all, but they'd want to help protect others of their faith if they can.
As freely as Hans was discussing things with the film crew, one has to wonder if he knew they would be disappeared by the government at the end. Maybe there have been others before them who got disappeared as well?
When told how trolls in fairy tales act like people, Hans immediately says that fairy tales are for kids and don't necessarily conform to reality. Well of course trolls would act like people in fairy tales and be wild beasts in real life; fairy tales also portray animals as sapient, when they really aren't, so why should trolls be an exception?
Why, in the first troll sequence, does the Tosserlad sniff around hesitantly, instead of going straight for Kalle? It's mentioned, shortly after, that Thomas sung in a church teen choir (because his parents forced him to), implying he comes from a Christian family, even though he personally does not believe in God. That's why the Tosserlad was hesitating so much, it didn't know who the Christian in the group was.