Don't the D pad target stuff? Pretty sure that's the normal way to target, other than X for nearest target, like left/right to switch targets iirc, maybe up/down it's been a while. Alternatively you could use some program to bind it differently? Not sure what you use for your pad if you're using native support or what, I think you can get it to go through Steam first to let you use a profile for your pad instead, or a 3rd party or autohotkey. Same with the mouse, can rebind any of the side buttons to tab or something.
Not 100% sure what you're trying to do though, play with both the mouse and pad at the same time? How do you use skills like that? If you're just playing with a pad, and using the mouse to click on menus etc, then any mode does that.
Maybe your issue is your targetting scheme is set on something bad? I forgot how to switch it on Gamepad but just above your healthbar, there's a small indicator that should say All, when it targets everything. You can set it up to only target allies or enemies and a custom one. If it's on allies for example it prevents you from targeting enemies when trying to turn in a quest or target a NPC to heal it, that'd also prevent you from tabbing. If you go into the targetting options, one of the tab has all that crap to select the right scheme, the binding to switch should be in there. It's probably some shit like hold L1 or R1 then press down/up or whatever.