Cover is currently extremely underwhelming. The best use for it right now is being able to redirect an attack to yourself when tank aggro won't work for that, assuming it either saves someone from certain death or making a mechanic intended for DPS (mainly Damage-over-time effects) go on you instead.

In your case, Tonberry stabs are pretty much non-lethal at this point, so your Healer could've just taken care of it.

Regarding controller usage for abilities, if you aren't doing well with the D-pad target switching, consider making targetting macros for party members and assign one crosshotbar layout for it.

There are plenty controller setup and macro guides out there that go into detail about it, feel free to look them up.

Using more Sheltron is fine, just make sure you don't use it unless you actually mitigate damage if you do.

Regarding your Silver Feathers, there isn't a whole lot you can buy with them, Ironworks gear probably isnt bad as it saves you a bit of Tomestone of Poetics farming.