Pretty sure that the only airline that flies SEA-CDG-ATH (the original ticket) would be Delta, or Air France/KLM. Delta/Air France are the only airline(s) with a nonstop from SEA to CDG.
But if they re-routed to SEA-SFO-MUC-ATH, then that doesn't make sense. Given the different routing, that would almost certainly be United/Lufhansa (only they fly SFO-MUC nonstop).
Maybe the original routing had another connection domestically before the leg to CDG....that would be how United (and their partners) would route it.
Honestly, these days, if you're flying to Europe from most places in the US and you do NOT have a nonstop to Europe, you're changing planes and will likely be on (at least) two different airlines. It's hard to avoid that from many places.
The value of nonstop flights has gone up this year. It has always been better to avoid connections if you can, now the stakes are higher.
Just because your bag has a tracker on it, and maybe you can even hear
it beeping from within a baggage cage, doesn't mean you can actually
get to it.
Indeed, simply telling someone where you think your bag is doesn't mean they're going to do anything about it. Trackers or no trackers, you are at the mercy of the bag handling folks.