Yeah, it is; gl_crontabs is for, unsurprising, the GL GUI side of things. It's better to use one or the other but not both.
They make some seemingly counter-intuitive development decisions compared to just hooking directly into OWRT. IDK why they seem so intent to reinvent the wheel.
Oh, probably. I haven't dug into too deeply. Anything I have running GL firmware I tend to treat as an 'appliance' vs getting real work done on pure OWRT.
I'm just not sure how much longer the GL firmware is going to be attractive to you. I think this sentiment also applies here:
... but this really going off topic. I'll back off now. TTYL, I'm sure.