I just visiting Rift, seems like I have to do the frame someone else quest to proceed the quest.
I really hate these kinds of quests.

is it mandatory to do them to progress main quest?
no shortcut?

You do not need to join the TG in order to get on with the main quest. Try talking to Keerava (the Lizard bint in the boozer), she should give you the information you seek.

Yeah, that part seemed a bit broken to me too and I had to look up the info online to find out you could avoid talking to the thief and just go to the bartender to update the quest.

No.

Not at all. Just go to Vexel The Man, the barowner Ragged Flagon and bribe him for the info.
Or go to Keerava.

The games rigged. Bethesda deliberately hid the none thieves guild options to force people to join it.
It says speak to the stupid b**tard brynjolf even when your escorting Esburn through the ratway and clearly have found him.
They were really, really freaking obbsesssed with forcing people to join that piece of s**t faction.

They pull a similar stunt with the Mages' College later in the MQ; you can't bypass it unless you metagame, since there's no way your character would ever know about it ahead of time.

For the curious: assuming you already know where it is, you can just go straight to Septimus' Outpost once you get to the point of visiting the College. He'll still give you what you need, as long as you do what he asks (which you want to do anyway, because the reward is incredibly useful).

It doesn't require metagaming. It just requires a character who isn't too concerned about rushing through the MQ and who likes exploring. I had a mage character who went to Winterhold after Riverwood (since you can find out about the mages' college from Riverwood) and once he'd joined up, he did a lot of exploring around to hone his skills (and also looking for the barrow that went with Brina's claw). Found Septimus by accident. Hadn't even given the dragonstone over in Whiterun at that point.

I did this today, as someone else said I think you can avoid being a dang thief by going to the bartender and drilling for answers.

I love how you guys get angry over the game "forcing" you to join guilds.

*next topic*

Dude, you just do that one quest for them, and you never have to see them again. I don't know why you would not want to continue with the theives guild though

True, you can find it via exploration, and my current character did so. He didn't go inside, though, as he doesn't yet know Septimus actually matters.

In this case, metagaming means 'acting on knowledge the character could not possibly possess at the time', and the whereabouts of Septimus, let alone that he's actually important to the MQ, are such an item of knowledge. Thus, even though you, the player, found his hideout, it would be breaking RP (and therefore metagaming) for the character to bypass the College.

For what it's worth, even though I tend to have an RP for each character, I often bypass the College on those that don't use magic; I tend to fudge it by telling myself that whoever I spoke to about the needed item told me about the nutcase in the iceberg.