Hey Arne,

Thing to remember about realms lore is that the lore itself generally represents either factual historical details, or general principles of the present day and state of things. The latter isn't an absolute to-be-abided-by thing, and in fact most stories within the realms are about strange or unusual occurrences, where the norms are altered in some way; as long as it's made clear that the unusual situation is unusual, that's just dandy.

Originally Posted by Arne
(1) There are apparently Drow mingling with simple human scavenger groups instead of killing them

In the present realms date, there are more drow interacting with the surface world than ever before, and more surface-dwelling drow than ever. However, drow that still serve their great houses are still generally only above ground to raid, or on very specific missions. This doesn't mean that a surface drow who broke away from their society can't just as easily opt to pursue a life of crime or villainy; their motives can be as varied as the motives of any other sentient being. The zhentarim in particular employ a lot of surface drow who have fled their underground caste and society, but still have the skill set suitable for the black network.

Quote
(2) Tieflings are not depicted as special or enigmatic, but as commonplace and simply 'humans with horns'

Tieflings are not enigmatic and special, particularly. They are people, just like any other people. They have lives and families, and work mundane jobs, and try to get on with their peers. Tieflings often face a certain degree of mistrust or prejudice, because they do bear the traits of demonic touch in their ancestry, and some people are predisposed to judge them based on that, but the reality is that individual tieflings are no more disposed to good or evil than any other race. Many teiflings are fated to a 'more interesting' life, especially if a fiend in their lineage knows about them and has an interest in them, but those are the stories that make adventurers, heroes and villains... same as for anyone else. Teiflings are not incredibly populous compared to most other races. They are never a majority or even a very strongly represented population in any population centre. They're moderately rare... In the game right NOW though, we have an active divergence from that norm, because of recent events in the realms lore - the descent into Avernus of Elturel, a city on the coast, has resulted in a greatly inflated number of tieflings being present - some who were transformed as a result of the descent, some who made deals of opportunity at the time, some who were coincidentally conceived (or just carried) at a particularly bad time, and some who may have had the blood legacy unexpressed, which was then brought to the surface by the events. There are more tieflings being born recently, thanks to that event, for the same reason that you often get plane-touched children being born around the times of planar incursions.

On top of this, Larian have decided (this is on them, and I don't at all agree with it), that the resulting behaviour from Elturel and other nearby cities has been on of extreme racism to the extent of driving tieflings out and making them refugees. That's the only bridge-too-far as I see it; that simply would not happen in the realms as they're described. There might be malcontents, and there would definitely be tensions, but no unified excommunications; many of these cities are prolific melting pots of different races and racial acceptance is the norm in them.

Quote
(3) An especially weird/funny moment was as two Tieflings were discussing how 'alien' Lae'zel looks while they are obviously looking much more alien wink

Not in the realms. Gith are far, far more alien and unusual than tieflings, by a long, long shot. People know what tieflings are, and recognise them. They may have prejudices or biases, but they also know that tieflings are a thing, and that they live lives and run businesses and work jobs. Most common folks, on the other hand (including tieflings, just as any other race is included here), aren't likely to know what a Gith is, or recognise them at all. They might think a gith a fiend of some sort, at a glance.

Quote
(4) Another situation was when I met a devil making an evil offer to accept his help getting rid of the tadpole - looking exactly like the Tieflings, but now suddenly being enigmatic and evil wink

Tieflings and Cambions are quite different; Cambions are half-breeds directly of fiend and mortal union, but they have all the powers and benefits of true fiends, and the potential to rise very high in the devil hierarchy. Their demonic features are usually much more obviously pronounced than tieflings - they lean into their fiendish heritage a lot more, in terms of appearance, and besides that, they have a palpable presence that will register to divine senses and other such spells as evil, where tieflings do not. Most Cambions can disguise themselves to blend in as they wish, and can mast their presence, but beings with special senses will still be able to pick them out, in most cases. In the game, this creature is clearly a Cambion, and not a tiefling, not by virtue of how he looks, but by virtue of what he does and the powers he displays.

Quote
(5) In the next moment, I again spoke to Tieflings who told me how weak and afraid they were of Goblins, looking exactly the same as the devil

Call this one a limitation of the current game; they shouldn't. The Cambions fiendish features should be visibly more pronounced.... but it does sound from your comments that you're not familiar with tieflings in general. They aren't new to the game lore; they've been in the game since 3rd edition, as a playable race (maybe earlier? I'm not certain).

Quote
(6) Another situation was meeting a mind flayer in the underdark, who, in the course of a few sentences, was introduced as good and living in the middle of a myconid spore colony

Yes; it's unusual, highly unusual, and it is depicted as being highly unusual. It's not unheard of, however. Illithids that break away fromt eh direct voersight of an elder brain by choice rarely have any desire to go back to it. In such a case, the creature must still survive, and the ways in which they devise to do so vary from rogue illithid to rogue illithid. I found the premise here perfectly believable, and was excited to explore it. I wish him well, and want to do all I can to help him find his alternate way. (I didn't want to take his ring - I feel like he needs that more than I do, and was disappointed that I didn't have the option to say that.)

Quote
If your Tieflings are just commonplace and another form of humans, they are boring and nothing special. If your Drow are just humans with dark skin, they are boring and nothing special.

People are people, and they come in all different shapes and sizes and flavours. They are all people and they are all different in their own ways. The realms have a lot of different races and they have propensities towards different things, heritages that give them varied outlooks on life, and cultural lifestyles and practices that differentiate them from one another significantly. This story has plenty enough going on in it already to make it interesting without needing to form a crutch out of fantasy racism to add drama. Larian have an addiction to fantasy racism already, and frankly, it leaves a pretty bad taste in my mouth. I'm hoping they'll move away from that and focus on the actually interesting elements of the fantasy story they're telling. If I want a story about racial drama, I can get that literally anywhere else; I'm here for a fantasy adventure.