The sequence of events dealing with the Keystone is ultimately unavoidable. If you're at the Temple and have yet to acquire the Keystone, then there are several steps you need to take. Go see the weapons maker (the man that needs Mythril; you may have stopped by his house only to find no one there earlier in the game), and he'll let you know that he sold the Keystone to Dio. Proceed to the Gold Saucer, where a number of events (which I won't spoil) will take place, and ultimately lead you to the Temple of the Ancients.
The Tiny Bronco can access the second continent at a number of points, allowing for easy access to both the home and the Gold Saucer. If you're unsure as to where either place is actually located, I recommend you consult the maps included under the FFVII section of EoFF.
