1) ी is used with feminine nouns whether singular or plural/formal.
fem. singular informal --> मेरी बहन बैठी है। (My sister is sitting)
fem. singular formal --> मेरी माँ बैठी है। (My mother is sitting)
fem. plural --> लड़कियाँ बैठी हैं। (The girls are sitting)
2) ा is used with singular masculine nouns.
masculine singular (informal) --> मेरा भाई बैठा है। (My brother is sitting)
3) े is used with plural/formal masculine nouns.
masc. plural --> मेरे भाई बैठे हैं (My brothers are sitting)
masc. singular formal --> मेरे पिता बैठे हैं (My father is sitting).
I'm American, middle-aged. I believe for most ppl like me, "in" is normal. "On" is acceptable in many situations but would be slightly abnormal, or what linguists call "marked." Being the less typical, "on" is useful for describing atypical scenarios. We may normally say, "here, sit in this chair," while a photographer might use "on" to emphasize the positioning rather than the general action of sitting. Duolingo has to be a little bit simplistic so as not to confuse ppl. They sometimes seem to use a marked English form for an unmarked Hindi sentence, prioritizing literal meaning rather than typical usage. It's the dilemma for any translator, and for language learning, it's the right move. Our English brains are going to take the path of least resistance and probably default to "kursi me". Seeing "sitting on" helps to jar our brains into noticing the difference and practicing correctly from the start.
Yeah, most likely. It probably also corresponds to small differences in perception about the nature of the chair and the sitting. Just Googling for some strings, "sitting on a kitchen/folding chair" is somewhat more common than "sitting in a kitchen/folding chair" but "going to be in the chair" is many times more common than "going to be on the chair." Thinking about this, it crossed my mind that if I happened to be talking about a cat, I'd probably use "on." As long as the Hindi preposition here is the natural one for ordinary human beings' sitting with respect to chairs in their customary upright position, all is well. I'm sure they'll add the options with "in" soon enough.
It's the diacritic of र.
From wikipedia: र r(a) as a first member takes the form of a curved upward dash above the final character or its ā-diacritic. e.g. र्व rva, र्वा rvā, र्स्प rspa, र्स्पा rspā. As a final member with ट ṭa ठ ṭha ड ḍa ढ ḍha ड़ ṛa छ cha it is two lines below the character, pointed downwards and apart. Thus ट्र ṭra ठ्र ṭhra ड्र ḍra ढ्र ḍhra ड़्र ṛra छ्र chra. Elsewhere as a final member it is a diagonal stroke extending leftwards and down. e.g. क्र ग्र भ्र ब्र. त ta is shifted up to make त्र tra.