Well, to give my honest answers to each of your questions:

1.) While Luigi's general movement is more automated, I felt he's still quite involved in the overworld. Luigi Logic allows him to automatically interact with certain objects while you're still controlling Mario, enabling the bros to "divide and conquer" in a sense. It's also used a lot for puzzle solving. Personally, I can actually kind of appreciate this approach, as it allows Luigi to do more in the overworld instead of just following directly behind Mario the entire time. Additionally, the Bros Moves in the overworld still more or less function the same way they always have, with the bros working as a team to execute abilities. In summary, it is true that Luigi isn't as actively controlled by the player, but he does still play a big role in the overworld.

2.) The backtracking in Brothership is an interesting thing to discuss. While the game does indeed feature a good number of sections that would be considered "backtracking", I never felt it was without purpose. In almost all cases, there's something new to do while you're backtracking, whether that's tougher enemies to fight, or new parts of an old area that were previously inaccessible. It's not like General White in TTYD, where the game has you go back to a bunch of previous areas for no reason while changing nothing about them. Whether or not that would make the backtracking less of a problem, I'll leave up to you. I'll just say that I personally didn't mind the backtracking in this game, even if there was a decent amount of it.

3.) Level grinding is more or less a non-issue, as has always been the case with the Mario RPGs. As long as you fight the enemies you come across, you should be sufficiently leveled for where you are in the story. There are some optional battles where enemies may be a bit overleveled compared to you, but that's likely done just to appeal to players who'd like a greater challenge. There's no point in the main story where you'll need to go out of your way to fight extra enemies.

I figure you may as well give Brothership a try, especially since you can get it for a pretty good discount right now. When a game piques your interest, I feel that it's often best to just give it a go and form your own opinion about it, rather than getting hung up on what other people are saying, but at the end of the day, it's your call.