What's the best way for Buddhism to spread to Europe and eventually become the dominant faith there? Bonus points if you can accomplish this post-Christianity and/or post-Islam.
That said, you can always have aggressive cultures that are nominally Buddhist and still conquer Europe and force it to convert (at least nominally), like a varaiation of the Mongols.
This is actually a scenario that's interested me - if nominally-Buddhist steppe empires like the Mongols could devastate Europe much more thoroughly than in OTL, could at least a portion of Europe become nominally Buddhist? I expect that the everyday peasant would continue practicing Christianity, but could we see post-Mongol ruling classes adopting Buddhism out of political expediency?
Love the Hellenistic route, btw. Such a world would be very interesting - maybe a strong Buddhist influence on early Roman civilization? Buddhist missionaries converting the paleo-Balkan tribes, perhaps Buddhism in some form even reaching the Celtic and Germanic lands?