As you can see, there are already comments that aren't too valuable here :D I'm going to try my best to answer all of these questions that you asked.

Concerning the comments, you only get something if you or others upvote them. Of course, there will always be people who are just commenting for the mere fact of getting upvotes, but if you don't have a lot of Steem Power, you will only be giving them cents. However, if you comment on people who are orcas or whales (50000SP and 500000SP), they can give you quite a bit. This is the reason why you see loooooooooooong comments on orca's posts or whales posts or even dolphins (5000 SP). So, yes, you are right that some people are just commenting for the sake of getting an upvote, but there are others who are not. I can only say that I've never experienced such an amazing and awesome community like this because everything is so new and everyone is growing with each other. Back to the topic, though, there will always be these kind of people who are just commenting for the sake of money. But, on the other hand, if you think about it, it's also kind of a good incentive for them to be engaging :D

Concerning promoting yourself and spammers...there's actually a website called "steemcleaners" where you would report these kind of people and get rewarded with SP. You do this anonymously as well, so that is awesome, in my opinion :)

You are right about the trending videos, that most of them are related to crypto and dtube and steemit and it's because of, like you have already said, promotional purposes, which then brings up the question if everything is based on MLM. It smells like it and looks like it as well. If you actually look at all of the cryptocurrencies, there are actually all kind of questionable because nothing is really regulated and their value is solely based on the supply and demand, but yet they are legal. It's like in a grey hat zone if you take a closer look, but I guess the time has come to this where you can value something that doesn't really exist and make money with it. It's just a matter of how we perceive it. How much value are we giving to it. So, back to the question if all of this is MLM scheme. Yes and no. Yes, because the more people come in, the more value rises with it and no because it's not really an obligation to invite people in to make money. You will still get the same amount of money, but only the value of STEEM would rise if more people come in. So, let's say you get $100 on dtube, then right before the payout it would drop to $75 because 25% goes to dtube because they take that to improve the site and IFPS, etc. then if you have set the payment to (50%/50%) (which is the best one) then you would receive 37,5 Steem Dollars and 7,5 Steem Power if STEEM was $5 worth and you would be getting less STEEM if it was worth more. Sorry, that I just explained you the payout method, but I think it's good to know :D and so yeah...you are still earning even if you are not inviting anyone in it, however, that doesn't answer the question if it's an MLM scheme? In my honest opinon, all cryptos are kind of MLM scheme if you think about it because it's just getting value by us. I do realize what MLM scheme is and you might be thinking to yourself that what I'm saying doesn't really make sense, but if you reeeeeeeeally think about it...bitcoin, for example, has it's value because yes, obviously, people invested in it, BUT! if they weren't intrigued by it, if they didn't see the value in it then they wouldn't have invested in it and so therefore they kind of gotted invited indirectly to it.

So, the value of STEEM comes from us. From the people asking for it. It sounds very...MLM...like...yeah, I have to admit that too and they are whales who are upvoting themselves and therefore also giving money to themselves...which is...hmm...not fair, but since STEEM has its value and people are giving the value to it, it does exist.

There will be people abusing this kind of system without a doubt...but there will also be people who will try their best to make qualitative content. Right now, the biggest vote you can get from is dtube themselves and they actually do it manually as well. And yeah...some things are really questionable, but I guess it's just a matter of the individual themselves.

I hope I was able to answer your questions man :)

Winny out...for now ;)