That's not how criminal procedure works.
Pursuant to constitutional law, you are required to start with the charges first, so the defendant knows what they are being accused of and can prepare a defense.
In some but not all jurisdictions, prosecutors may offer "plea bargains" that reduce the sentence, or reduce or drop some of the charges, in lieu of proceeding to trial. In some jurisdictions (i.e., large portions of Texas), there are no plea bargains; you always go to trial.
Maximum sentence is set by law, as a matter of constitutional rights. Thus, your solution would simply restrict plea bargains to not being any shorter than 1/2 of the maximum sentence (in Aaron's case, 16 years).
Making the counts non-severable is a legal and constitutional impossibility. Double jeopardy requires the prosecutor to charge you with all possible crimes arising from a single act (or intertwined set of acts) in the same criminal proceedings. Once those criminal proceedings have been adjudicated (i.e., a guilty plea, a conviction, a not-guilty verdict, or charges dismissed with prejudice), the prosecutor cannot ever charge you again for any other crimes arising from that act. Your solution would either require them to choose which charges to pursue and which charges to drop forever. That's fine in cases like Aaron's, but that's not something you want when you're dealing with a rapist or a murderer (i.e, cases far more common than Aaron's).
Finally, the last suggestion, is not simply politically unpalatable, it is economically impossible. Most criminals are actually guilty; making the prosecution pay for every defense would increase the costs of the criminal justice system 1000fold. A better solution: make the prosecution pay the defense costs where (a) a defendant that is found not guilty or actually innocent, (b) a hung jury trial, if the prosecution does not refile, or (c) the prosecution drops charges before trial. This would accomplish what you want in a realistic manner (and is actually already the law in some jurisdictions).