i) Do you believe in every sutta?
ii) Couldn't it be the case that there are also many 'later alterations' to fit the dhamma to one's own/to culture's liking?
Yes. And also to create a religion that appeals to more people.
iii) Connected to question i): How do you interpret devas?
Devas are the rich, powerful or angels (people with physic powers).
And how do you verify 'post-morten' rebirth?
It cannot be verified. Most teachings about 'rebirth' are not "post-mortem" but the later teachings about 'rebirth' are. The basic teachings about rebirth are about results of actions.
Isn't it in contradiction of anatta?
No. The teaching of anatta does not mean 'self-view' does not arise in the minds of ignorant people. Thus 'rebirth' is the re-arising of self-view due to results of kamma.
Didn't the Buddha teach that the Dhamma can be experienced in the here and now?
The Buddha not only taught this but also defined his Dhamma as this. Therefore, anything outside of here & now realisation is not really the Buddha's true teaching.
Why do the 4NT (and N8P) not mention rebirth?
Because the Buddha's true teaching is not about "rebirth". MN 117 explains there are two sorts of teachings: (i) the true teaching for liberation and (ii) impure teachings for morality. This is pretty basic stuff for those with faith in the Buddha (rather than for those who get lost in social media Buddhism).
iv) Most people do not translate suttas by their own. They take the words from scholars and monks, which often copy from each other. Is
this the critical thinking that Buddhists often speak of?
In many suttas, particularly to Brahmins, the Buddha condemned blind faith. But later Buddhism became exactly what the Buddha condemned; probably because the later Buddhist clergy attempted to covert as many people as possible.
The above said, these issues should not concern a noble practitioner. These issues are the way of the world. Every religion gets twisted. For example, as soon as Mohamed (PBUH) passed away, this companions were squabbling and making up teachings. In Islam, there are literally millions of 'hadiths' claiming to be utterances of Mohamed. Then in Christianity, there is a multitude of varying scriptures. And the Hindus keep adding new stuff but claiming it is 1000s of years old. Why would Buddhism be any different; with writings like the Jataka Takes, claiming the Buddha was different kinds of animals in past lives that performed miraculous feats?