For most things having a single box sprite like you do for your platforming object is best, but it's also good to know when to break the rules
If you make a 180 angle for your platform object, then you can detect which direction it's facing. Just make sure the hotspot and size of the sprite are exactly the same as your 0 angle. In fact, you can just copy the whole frame over by selecting the 0 angle's frame with C and then pasting it into the 180 angle's animation with V (not Ctrl-C and Ctrl-V for some reason ).
Also, are you intending to have player control over the actual thrown object, or do you want to just wind up and let it go do it's own thing? If you want it to behave more like an actual thrown object without any further input from the player then you'll need to gather all the speed/force/angle data that you'll need prior to spawning the grenade and once it's spawned the rest needs to be totally automated. The Ball behavior might work for you in this case. Set all of it's information in the same action that it's spawned in. That's all the amount of player input it should get from that point on.
I'd work up an example for you but I just reformatted my hard drive earlier so my computer doesn't have Construct on it yet