Often, English prepositions function more as adverbial complements, forming phrasal verbs such as to "talk about." You're correct, if you avoid ending the sentence such "prepositions," you might sound like a weirdo. But that is something into which we don't need to get. ;)

There's a few issues here. Some phrasal verbs are separable and any pronoun must split the verb from the particle; other phrasal verbs take no object at all. So you must end the clause with a preposition in both these cases: e.g. the plane took off I took it off (e.g. my shirt). (Alan from 2.5 men is funny about "flip off" but wrong!) Many phrasal verbs are true phrasal verbs as opposed to prepositional verbs and are not separable. e.g. to look out for. You can't divide the verb from the preposition(s). So you would have to write: "I don't know whom/who to look out for" In other phrasal verbs, you can move around the preposition (like in your examples). The received wisdom nowadays in the UK is that it's not necessary to move "movable" prepositions in prepositional verbs from the end of a clause because it sounds too formal and awkward. As for prepositions when used normally (i.e. not in prepositional / phrasal verbs), then moving them away from the end of the sentence, like in your examples, is good style for formal documents but otherwise too formal. Sorry it's not an easy answer!

It's not clear what you're referring to. Did you notice it from someone you were talking with? If you give us an example, we might better understand what you're talking about. :)

I'm not sure what you mean, Javi. Can you give an example of a sentence you think is correct but which might also sound weird?

It could be