2. There is no legal scope for halala. After the third talaq a couple cannot re-marry, unless the woman has re-married someone else and that marriage has come to a natural conclusion. You cannot engineer a short term marriage with the intention of ending it to then remarry the perso you first divorced.
2:30
"
And if he has divorced her [for the third time], then she is not lawful to him afterward until [after] she marries a husband other than him. And if the latter husband divorces her [or dies], there is no blame upon the woman and her former husband for returning to each other if they think that they can keep [within] the limits of Allah . These are the limits of Allah, which He makes clear to a people who know."
As posted by @I.R.A
Cheers, Doc