1) it has been said several times that the Time Ring cannot go into the past. The only reason Black could was because of Trunks' time machine, and even then he was pulled back after on a few minutes. He also showed no real interest in the past and was content to leave Trunks trapped there while he finished off the rest of the Earthlings. We also don't know where Black came from, or even what timeline. He could be from the past, but still the future of the current timeline. And it's an assumption that he's even from the future to begin with.
2) Given that Black has a beef against mortals and not the planet, it isn't hard to believe that he's trying to do as little damage to the planet as possible while wiping out all the Earthlings. And he tried to kill Trunks several times and finds his presences an annoyance. The only reason why Trunks isn't dead is because of his quick wit and Mai saving him. It wasn't out 'I want a good fight' or 'this guy is fun'. Trunks would also have experience ducking and weaving since he survived the androids, and no one in the main cast has that. The one time they faced a situation like the androids was Majin Buu, and they huddled on the Lookout hoping for Trunks and Goten to learn the Fusion Dance.
3) Whis and Beerus never showed they gave two shits for anyone outside of Goku, Vegeta, and Bulma. Why would Whis train Goten, Trunks, or Gohan? I know fans love the idea of Whis training their favorite characters, but Whis doesn't train fodder. Vegeta had to beg and bride to even get Whis' attention. He even refused to train Goku when he asked because he was upset about all the time travel. They even left before they figured out who Black was.
As for Beerus and Whis saving the Earth like Resurrection 'F', given that Black didn't care that Beerus was hanging around when he fought Goku, I think he knows Beerus and Whis won't interfere, or he believes he can handle them. Also, Whis only rewinded time and allowed Goku to kill Frieza before he could blow up the Earth. Neither Whis or Beerus directly did anything against Frieza. If no one can beat Black directly, I doubt Beerus or Whis would jump in.
1) Excuse me for repeating myself. The time ring can only go to the future, and COME BACK to its original time. If he used the ring, and is in Trunks' future, that means he is from a time BEFORE that. Aka THE PAST. I never said he could freely travel back in time . You don't know when he departed from. If it's only a few weeks after Zamasu met Goku for instance, it explains why he wouldn't try to go back since he was just following Trunks' trail. I don't even see why you're trying to deny this is a possibility. And since the ring was in a sealed box, it is very likely that he did use the ring to get to Trunks' future. Therefore he can theoretically go back in time to his time of departure. That's all I was saying and that's an objective fact so I'm not sure why there's an argument here.
2) Yeah K, so Mai and Trunks managed to survive but nobody in the present could, according to you. Right. They made Mai survive just because they wanted to, they don't really care about consistency or "likelihood" of survival. She is STILL not dead precisely for that reason. The androids could not feel ki so I don't think the Black situation is similar for Trunks.
Likewise Black couldn't feel the ki of 17 and 18 in the present, Krilin and Gohan also had quite the experience on Namek. Trunks and Goten have proven to be rather sneaky.
Since as you said, Black isn't gonna destroy earth, they're gonna survive for a while. If shitty humans can have a resistance in the future despite how broken the world was, I'm sure present earth can, especially if helped with stronger allies. If anything it would still take Black a long time to ruin the world.
3) Whis does whatever Bulma wants if she has a convincing offer, though. Bulma is still in the present, if she asks him to help and he refuses to intervene directly, she could ask him to train the kids or something. This is far from "impossible". Assume everything Vegeta knows about Whis' world and "room" to have been shared with Bulma during the various time skips.
So you're telling me they're annoyed by Goku because of the time travel yet would be okay with an asshole coming in and destroying a world thanks to time travel ? Yeah I'm gonna pass on that logic.
They did what they could to figure out who Black was, they virtually have no leads. They followed the one trail they had. Whis and Beerus did mention that Zamasu wasn't connected to Black "at this time", there's not much else they can do.
And as for saving earth in FnF, NOTHING forced them to. They acted on their own will to help earth. It doesn't matter if they killed Frieza or not, rewinding time is made out to be a big deal. In Super, in the aftermath, Whis comments that Beerus is warming up to the earthling because he allowed him to rewind time.
I don't see how you could think for a second that they're going to let earth get ravaged by Black, since it means all their previous food also gets ravaged, while also violating time travel rules, a thing they are very annoyed by.
But I guess it comes down to opinions, right ? So feel free to believe what you want, I will.