How would one prevent Ottoman conquests in Europe proper? What would need to happen in order to stop this?
Prevent the Byzantine civil war, for one.
Reverse the Turkic migrations away from Anatolia, and thus butterfly Ottomans entirely. So long as they end up there, they WILL be into the Balkans eventually.
Prevent the Byzantine civil war, for one.
This is literally it, which can be solved pretty easily. Either Andronikos III doesnt die or one faction cements control before war breaks out.
Nothing's inevitable, mate. Indeed the Ottoman incursion into Europe started because Gallipoli fell to the Ottomans because of an Earthquake.
Hell, a PoD at 1261 could easily take care of the OP's demands. Have another Turkish tribe gain prominence and have THEM take the Balkans.