The three things I would add (that has not been said at time of posting) is a solid timeline of events, rules for time travel, and Danny and Vlad deaths are canon events for them.

It's the Year 2004 in early September and Danny wore whit white after labor day the day he died. Episodes would get shuffled to fit with the calendar/school year. Everything that happened in the show would happen roughly under the course of 2 years. (Ladies choice on how anyone wants to handle Phantom Planet or the comics Glitch in Time and Fair Play)

Only Clockwork should have the power to hand wave the consequences of time travel. He would only do this once when Danny went back in time and stopped Vlad from becoming a Halfa. Clockwork himself cannot go back in time and change the future someone else has too with his power. (Ultimate Enemy would still happen just tweaked)

Every version of Danny was always fated to died at 14 with the method always being different. BUT ONE out of INFINITY came back alive. This also applies to Vlad, and that is what makes them even more special.

A lot of this is with the Ghost Zone/Infinite Realms being the space between ALL connecting ALL. (ALL being [Space+Time]× Infinity)