Can I get a 4* +SPD -HP Tharja to work?
Absolutely. Be sure to rank her up to 5 if you like her.
Now that we're on the subject - while I probably won't be actually using Ike much, how should I go about salvaging a -SPD +ATK nature? Should I be going after Quick Riposte? Is a Threaten skill worthwhile?
If he's your only Ike, then there's no harm in trying. You literally just need Quick Riposte (QR2 should be fine) to ORKO most enemies on enemy phase. Triangle Adept should significantly limit the amount of damage you take against Greens, even if they can double you. This way, you hard check Green Mages as well. Threaten Speed would be nice, but it's not that big of a deal. You also definitely want to run Hone Speed users to make up for the Speed loss.
95% of the time, IVs don't make that much difference. And honestly this is one of the most F2P friendly gacha games I've ever played. If you have to have specific characters with specific IVs, then of course you're going to be disappointed. But you won't need that to do all the content or to win in arena.
This!