I can't download Dying Light because once I get to 90% completion, it throttles my download speed down to 0.5 MB/s. Before 90%, my download speed lingers around 10-20 MB/s. I would use the back-up offline installers, but it says online features won't be enabled, which i plan on using multiplayer. I bought the game just to play with a friend and I can't even download it.

Who said so? Of course you need Galaxy installed and launch the game though it if you want to play online but it shouldn't matter if you install the game through it or not. As far as I know Galaxy has a function to import games installed through the offline installers so you should be fine to download an install through them.

Yup! In fact, Galaxy should automatically recognise any game installed off a recent offline installer.