Everyone else that we know has time-traveled has told MC about it; Skylar hasn't. I don't take her as being malicious (I hope not; to this point she seems like a sweet girl), but perhaps being manipulated and/or coerced.The question is where did she go and who sent her there, our team or the bad guys from the plant?
We really don't know about either. The old machine sent MC back 20 years, and then returned him, he having been gone from 2019 for the length of time he was in 1999 (IOW, exactly 20 years each way). Could it have done otherwise? We don't know, and I don't think we're given enough information to even make an educated guess. The new machine, as far as we've seen, has done the same. Can it do otherwise? Again, we don't know. We see other time-travelers, and it appears that their travel hasn't followed what's thus far been the norm with MC. If they've been using the same machine, that means it isn't restricted to working in that way.this new machine he used with Darci seems to be different and could have other rules.
Now, if you grant the existence of a time machine, it seems more than a little weird to have designed it to only work in the way MC's seen it work--to only move people back and forth by exactly 20 years at a time. That alone suggests it can do otherwise; we just haven't seen it yet. I think we will, though.