So I'm right outside of Temporal Tower, and I am totally unprepared, and I need to go back to Treasure Town to train. If I actually go back to the entrance of the Hidden Land, will I have to go through the Hidden Land again if I want to go back to Temporal Tower?
I'm sorry but it looks like you have to go through Hidden Land AGAIN if you want to go back to treasure town and get ready again. I've looked at two sources and they say the same thing:
Pokemonwikia says:
The only way to gain access to this dungeon is to obtain transport to Hidden Land by speaking to Lapras who can be found at Treasure Town Beach. The player must then negotiate their team through Hidden Land in order to reach Temporal Tower.
Bulbapedia says:
The only way to access it is by taking the transportation from the Hidden Land.
In other words, both mean that you must go though Hidden Land in order to reach Temporal Tower, since Lapras does not take you to Temporal Tower's entrance directly.
My suggestions~
I hoped I helped! Sources: Bulbapedia: http://bulbapedia.bulbagarden.net/wiki/Temporal_Tower Pokemon Wikia:http://pokemon.wikia.com/wiki/Temporal_Tower