Prosecution is discretionary. We cannot guess about how the prosecution would decide to exercise their discretion.
However to address your concrete points:
- Crimes do not "balance out" or "make a right." Every crime can be independently prosecuted. It may be the case that one defendant argues that acts of another justified or excused their act.
- Bob has not "cleared himself" (see Code 2.3 below). The evidence may make prosecution on sexual assault less likely (see Code 4.6 below).
- The prosecution cannot "ignore" any allegation (see Code 3.6). It must make a prosecution decision. By internal policy, this must be made in accordance with the Code for Crown Prosecutors.
- There are no "slam-dunk" cases. There is always the possibility of unexpected evidence that might change the assessment of the strength of a case.
- There may be more or contradictory evidence that makes what seems clear at first less clear in context.
Some important principles from the Code:
2.2 It is not the function of the CPS to decide whether a person is guilty of a criminal offence, but to make assessments about whether it is appropriate to present charges for the criminal court to consider. The CPS assessment of any case is not in any sense a finding of, or implication of, any guilt or criminal conduct. A finding of guilt can only be made by a court.
2.3 Similarly, a decision not to bring criminal charges does not necessarily mean that an individual has not been a victim of crime. It is not the role of the CPS to make such determinations.
2.4 The decision to prosecute or to recommend an out-of-court disposal is a serious step that affects suspects, victims, witnesses and the public at large and must be undertaken with the utmost care.
3.6 Prosecutors review every case they receive from the police or other investigators. Review is a continuing process and prosecutors must take account of any change in circumstances that occurs as the case develops. This includes what becomes known of the defence case, any further reasonable lines of inquiry that should be pursued, and receipt of any unused material that may undermine the prosecution case or assist the defence case, to the extent that charges should be altered or discontinued or the prosecution should not proceed. ...
4.6 Prosecutors must be satisfied that there is sufficient evidence to provide a realistic prospect of conviction against each suspect on each charge. They must consider what the defence case may be, and how it is likely to affect the prospects of conviction. A case which does not pass the evidential stage must not proceed, no matter how serious or sensitive it may be.
4.10 It has never been the rule that a prosecution will automatically take place once the evidential stage is met. A prosecution will usually take place unless the prosecutor is satisfied that there are public interest factors tending against prosecution which outweigh those tending in favour. In some cases the prosecutor may be satisfied that the public interest can be properly served by offering the offender the opportunity to have the matter dealt with by an out-of-court disposal rather than bringing a prosecution.
