checking baggage

I would think if it is all on one ticket, they should check you baggage through to Athens. It it's on separate tickets SEA-AMS and AMS-ATH, you probably have to pick up you bags in AMS and check in on the second ticket. But call the airline. No matter what we say here, they have the final say. When you check in in Seattle, if the baggage stub says ATH, it will be checked to Athens. If it says AMS, you'll have to recover it in Amsterdam.

Us small town folks are use to flying the puddle jumpers like Horizon to InterCon gateways like SEA. Interestingly enough, my InterCon gate way is often SEA with DL flying SEA-AMS route. At times I would hop from MFR to PDX to SEA or the reverse just to leave the country. Strange that Delta is sending you to PDX.

Given that Horizon/Alaska partner with Delta, you should have been able to get a Delta award ticket including your Horizon DEA-PDX. That would allow you to check one bag with no extra change in addition to your carry-on + personal. The checked bag should be checked from SEA through to your final destination. A 3 hour connect at AMS should be a big margin for both you and your bag.

Note that Horizon flies small aircraft and allows/encourages you to self gate check your bigger carryon with their green AlaCarte tag. You place the bag on their AlaCarte trolley on the airfield apron next to the aircraft and got on. The baggage handlers load the AlaCarte carry on into the cargo hold after checked bags are loaded. On arrival, the handlers offload the AlaCarte carry on and place them on a similar trolley for you to retrieve on off loading and before you enter the terminal.

I would not anticipate any problem with Alaska checking the bag to Athens. I've certainly had my bags checked through several an AMS transfer many times. I also (with some effort) convinced a Kos island agent to check my bag through Kos-ATH-SEA, after pointing out Olympic's code-share with KLM.

Unless you're old and unable to pickup your luggage, never, ever check a bag. Limit your wardrobe to where it'll fit into a 22" rolling bag. See Rick Steves' packing list that you can travel indefinitely with.

You'll never lose a bag if you refuse to check one.

I went against my travel rules and checked a bag in Prague April, 2014. And the bag had my car keys in it--my mistake. We sat in an Atlanta airport hotel for two days until our bag finally showed up. Never again!