You have to infer the answer to the first question from NFHS 8-3: "The free throws awarded because of a technical foul may be attempted by any player of the offended team, including an eligible substitute or designated starter. The coach or captain shall designate the free thrower(s)."
In order for "an eligible substitute" to attempt the FTs, that substitute must be able to enter the game before either the first or second FT. And since the two free throws may be attempted by different players, it may be required to allow a substitution before both the first and second FT.
As far as I know the rules book only specifies that each team must occupy the bench designated by game management and that they begin the game shooting at the basket farthest from their bench. I know of no rule that specifies that they must warm up at that same basket, although obviously that is how it's done. There is no rule, in the rules book at least, that says that a team cannot warm up at both baskets if the other team leaves the floor. And it's generally accepted that if the rules don't say it's illegal, it's legal. However, I believe the NFHS had something to say on the subject a few years ago, at least in regards to prohibiting teams from running through the other teams warmups, circling the floor, competing over the center circle, etc. And I understand some states have policies on the subject as well. So, in your state, this practice may very well be prohibited.
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"It is not enough to do your best; you must know what to do, and then do your best." - W. Edwards Deming
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