No new quest dialogue for College of Winterhold questline?

I think both anomalies are unrelated.

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The first one - Mirabelle not having the appropriate dialogue option at the end of 'Good Intentions' - is a known bug:

You may not get the next quest "Revealing the Unseen" after completing the quest. This appears to be permanent and cannot be fixed by reloading. As the quest does not load, Mirabelle will have normal dialogue and the quest is impossible to begin.

This (as well as a tremendous amount of other bugs) is fixed by the Unofficial Patch.

If you're playing on PC, you can try running the console command setstage MG06 20. This should skip the part where you have to talk to Mirabelle, and give you the directive to "Find the ruins of Mzulft".

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As for your second strange occurrence: I don't know why he would be dead.
Turrianus (he indeed is enchanting, isn't he?) is a protected NPC, which doesn't mean he is unkillable, but that he's only killable by the player (or everyone during bleedout). Apart from a rare bug, you accidentally might have been the cause of the poor guy's demise.

In any case, if you're on PC, you can resurrect him through the console. Mind that this might cause other bugs, so use it with caution.
"There are a few ways we can do this, and the choice is yours":

• By clicking on his body while having the console opened, and using the command resurrect. Be sure you clicked the NPC, and not some other entity.
• By repeating that process but using recycleactor instead of resurrect. Recycleactor resets aspects of the NPC, instead of just adding health like resurrect does. This seems to be the less buggy option.
• By resurrecting him remotely, using resurrect 0001C23F. To see if that worked, you can place the NPC right in front of you using the command prid 0001C23F, either before or after resurrection.

You can create a quicksave before checking whether or not the command worked, and if it did, reload it and play from there on out.

More on the specifics here.