You open the portal cheats in the settings.
And as for the beach area, the resort has a map of its own, which you can access via the map button. That's the same as the main area.
The bug about sleeping alone there, yeah, bit weird >.>
I don't think you can get MC to be thinner than what she is initially at current time. But if you do want her to not gain weight, then just make sure she only eats small things when the marker of when she is hungry pops up.
And right now the characters who can inflate with water can 'overinflate' and throw up/run to the bathroom if you let the mark go past the limit. That does make them regress back to a previous stomach inflation stage.
Otherwise, just gotta wait for them to decrease naturally.