Not sure what you mean by completely different thing, the other person was talking about new players being left behind stuck on old low level content, the mercenary system is designed for the sole purpose of helping with low level content like dungeons along side the fact there is a lot of other elements which makes worrying about new players and old content less worry worthy.
I have no idea where you go that idea, it will be gated for all both prior and after launch of 3.0. There is no evidence it will be limited to anything related to pre-orders even in the links you provided. What we currently know is Ishgard is gained access through completion of the main quest, we know this quest will happen after completion of previous main story quest line related to Leviathan and Shiva which highly references Ishgard. We know the new jobs require being level 30+ in one of the current classes, we do not know where the new job quest lines will start but logically we do know it will start in a location that is safe for level 30 class characters to be.
I personally have not see evidence that Ishgard is just a safe bridge distance from Coerthas so I have no idea if that is the case or not but since we do know the quest itself required for unlocking the new zones is level 50+ then there would be no point putting a level 30 job starter NPC behind that locked content.
What makes more sense to me is you level your character or current class to 30, at that stage you can start the new jobs, that new job NPC quest line will need to start in a zone fitting for a level 30 character to be and we know you cannot gain access to the new expansion locations until after level 50 so logically that job NPC will be located outside of Ishgard. What will then probably happen is you will level that job from 30-50 going back and forth to that NPC to gain skills and more including continuing with MSQ as that job but after hit 50 and gained access to Ishgard that job NPC may tell you to go to Ishgard to finish your training where learn the skills between level 50-60. There may well be a guild relating to that job in Ishgard but it would be extremely unlikely you would start the job quest line there when we know the location will be gated behind being level 50+.