http://www.kiya1.de/images/Mttquest.html

the correct sequence - which version are you playing? Pedro in the sewers only turns hostile after you got the tip from Tingalf about the sewers in version 1.0034.

Please, read the sequence in the link and maybe talking to the merchant guild leader in Verdistis might clear a bit of your log - after that talk to Robin again.

In versions prior to 1.0034 the mess was worse. But even installing it now wouldn't help, if you have started the Marcus quest already. I can only think of reloading before you spoke the very first to M. - or know where you went wrong in the sequence.

It's this:
Marcus - waggon/Luther!!! - Marcus/wheel - burning waggon/corpses/cave/letter - Marcus - Tingalf - sewers/letter - Robin/2 speech options - Marcus - Trevor.
Kiya