Emm.. there is no "loop".
The Admiral Janeway v.1 came from a different and alternate timeline where it took the crew, a total of 23 years for Voyager to get home. Now due to the alteration of the timeline of 2378, it meant Voyager has got home in 2378 instead. Therefore Admiral Janeway v.2 will not have to go back in time. (SGA had a similar episode - (1x15) "Before I Sleep"). The writers made a slight error, they wrote the bit where the Queen says, if Voyager was to be destroyed in the Transwarp Hub by the Borg Sphere, somehow it will erase Admiral Janeway v.1.
Not true because the timeline has been changed the instant Admiral Janeway v.1 appeared and redirected Voyager's path. In original timeline, Voyager didn't enter the Transwarp Hub, now new timeline, Voyager has entered the Transwarp Hub + gained system upgrades. Now if the Borg went back in time to the point before Admiral Janeway v.1 shows up i.e. before a temporal rift appears and the Borg goes and destroys Voyager, then the whole timeline would be changed again and in this new timeline, Admiral Janeway v.1 would not have existed, as Voyager would have been destroyed in 2378 instead.
Due to the introduction of the Regenerative Ablative Armour, Transphasic Torpedoes and Stealth Technology in 2378, it will further strengthen Federation's arsenal, however will be ineffective against the Borg.