Tracking is mentioned only briefly, in/around the pages you mentioned, in both the PHB and the DMG. And as you pointed out, in the PHB and in the DMG it doesn't specify tracking as having any maximum speed.
On PHB page 181, under Travel Pace:
A fast pace makes characters less perceptive, while a slow pace makes it possible to sneak around and to search an area more carefully (see the "Activity While Traveling" section later in this chapter for more information).
This sentence, depending on how you read it, could mean that any "Activity While Traveling" (which is the section Tracking falls under) can only be taken while moving at a slow pace:
... a slow pace makes it possible to to search an area more carefully (see the "Activity While Traveling" section ...
However, several activities specify how they operate under different speeds, meaning this interpretation doesn't hold up.
Additionally, Foraging, another Activity While Traveling that points to full rules in the DMG, has the following statement (DMG page 111):
Characters can gather food and water as the party travels at a normal or slow pace.
It makes sense that Tracking would have a similar statement, but it does not, so for the time being there's no RAW restriction on tracking at a fast pace.
You can, however, interpret it as being RAI due to Foraging or due to the negative implication of the Barbarian ability.
Yes, these abilities should stack fully. While previous editions had rules around stacking multipliers, 5e doesn't, and even if it did, Powerful Build doesn't directly multiply your carrying capacity - it just makes you count as Large.
Your carrying capacity as a Str 20 character = 20 × 15 = 300.
Then being a Large creature (from Powerful Build) doubles your carrying capacity to 600.
Finally, the Bear Totem feature doubles your carrying capacity to 1200.
This isn't particularly imbalanced - you had to work reasonably hard to get it, and carrying capacity doesn't matter that much anyway. Besides, it's not enough to let you beat the Elephant :(
Best Answer
From the Player's Handbook, page 147, in the descriptions of weapon properties:
Carrying capacity has no effect on your ability to use heavy weapons without disadvantage. That's determined solely by your size, and since this effect doesn't change your size; no, you still can't wield those weapons effectively.