It could mean several things. It could mean that the native speaker is poorly educated and doesn't speak very correctly. It could mean that they are speaking in a way that is typical of the casual usage in their locality. It could mean that they are playing with the language in a poetic way. But it probably doesn't convey any difference of meaning in the words.

It is technically correct, but unnatural. The more common expressions would be "I am not as tall as you" or "I am shorter than you."

Yes, they could both mean the same thing, but not necessarily. The second one sounds a bit more natural, probably because it clearer. In the second one, it is clear that the total time you spent on your homework was less than 3 minutes. But in the first one, while the most natural interpretation is the same, it leaves the hearer wondering whether this 3 minutes "finishing" the homework was after when you returned from a break after a few hours of working on the homework. Technically, both of them could be used in that context too, but the second one is unlikely to be heard that way.

Probably the first one. It sounds more likely. But it may depend on the requirements of a particular organisation, so I can't be sure.

They are both okay. Nayan's description of the word "activity" is technically correct, but most native English speakers would not be so strict about it. It would be quite common to hear the word "activity" used to describe something bigger that contained many smaller activities. So both are okay, but there is a subtle difference in emphasis. If you say it was a "great experience", you are emphasising it personal impact on you. If you say it was a "great activity", you are probably making a more objective statement about the value of the activity in achieving certain goals.

Yes. You can say all those things and you will be understood as you have described it. Good work!

Either of them is okay. The second one is probably better, but the first one is a very common abbreviation of it.

You don't need to use "this" or "that" in any of your examples. You use "this" or "that" to distinguish something from other things, e.g. "Not just any of the pens, but this one." In all your examples, your opening line has already made it clear which pen, present, people or friend is being talked about, so the lines that follow can just use words like "it", "they" and he or she. The sentences could still be grammatically correct if you use "this" or "that", but they will not sound very natural.