I'm doing something similar currently with a dry mead (13% goal, 6 gal) using ec-1118 at 64-ish degrees F. It's been fermenting for three weeks without signs of slowing down. I'm going to guess 6 weeks before I can even think about bringing it to secondary.
I've you're getting a slow start due to pH and fermenting low (60-65deg), it could take longer. If you're fermenting warm (68-74deg) it could go slightly quicker. Be wary of the acid content without any sweetness to counterbalance, as this could take quite sometime to come around.
And to clarify, potassium sorbate inhibits yeast reproduction, but does not stop the current culture (dry finish). Potassium metabisulfite will stabilize the product and probably stop the current culture (semi-dry, depending on when you add it).
Current opinion on leaving things in primary for a month is "no noticeable difference". Letting beer sit on lees (yeast hulls) for a long enough time allows autolysis and may produce a chemical undertone. Some say letting a chardonnay sit on lees creates a buttery flavor. However, this is due to malolactic fermentation which will probably not occur in a honey only mead. Your addition of pineapple may allow malolactic fermentation (don't add potassium salts!) during an extended rest in primary (but could happen in secondary as well).
**EDIT**
Sorry. I added all of that without providing much guidance. If you proceed with your current recipe, I'd say just let it go and figure out later what worked and what didn't. I personally have had many bad batches but don't mind keeping them around and revisiting as they age. (It's a learning opportunity)
If I were to make changes to your recipe, it would be this:
Do a honey only primary and then add the fruits at secondary to preserve the fruit flavor. I'd wait for the end of primary ferment and age a few months before moving to secondary. (transferring to secondary with fruit immediately will allow the yeast to attack the sugars in the fruit) Additionally I would add the fruit only at the peak of ripeness, as much of the flavor in pineapple and mango comes from sugar. Imagine the flavor of the fruit if you were to take out all of the sugar. That is similar to what would show in the mead. The pro-tip is to wait for the fruit to be ripe and then freeze it. Freezing it allows you to save it until you need it (of course) but also breaks down the flesh and thus more immediately macerate in the mead. Take note that you will end up with a lot of mush to strain out of the secondary if you freeze the fruit.
Good luck and have fun!