In order to establish the possible criminal liability of Joe and Keith it is necessary to identify the possible offences and the elements of those offences to determine whether either or both are culpable of the respective offences. Looking at the scenario as a whole the area of law we are mainly dealing with is homicide, but I will explore and support the possible offences in order of chronology.
In this situation Keith elicits help from Joe to kill his wife. They discuss a fee and a means of payment. At this point both Joe and Keith are criminally liable for the statutory offence of conspiracy as set out in Section 1(1) of the Criminal Law Act 1977. The ultimate intended consequences of their agreement was that if their 'conduct shall be pursued' (Criminal Law Act 1977) it will end with 'commission of the offence' (Criminal Law Act 1977).
The 'agreed course of conduct' (Allen, Michael Textbook On criminal Law, 2007) principle was supported in R v Siracusa (1990) and it also established through their method of interpretation that as with this scenario the defendant would not necessarily have to or intend to play an active part in the actual commission of the offence. Up to this point there was some confusion as the criminal liability in relation to conspiracy when one part did not intend to play an 'active role in the commission of the substantive offence'.
Now however following R v Siracusa (1990) and the 'highly persuasive' Privy Council, Yip Chiu-Cheung v R  cast further doubt on Lord Bridge's dicta in Anderson . It is now considered that participation 'can be active or passive. However Anderson  is a House of Lords case, and although due to the privy Council ruling it is likely that if a similar case reached the House it would be distinguished it is still good authority. Another factor is that it does not matter whether or not the 'commission of the offence or any of the offences' (Criminal Law Act 1977) are possible or whether the offenders go on to commit the offence.
They are criminally liable as long as there is an 'agreement between two or more parties' and communication of this, and the supposed consequence are in 'accordance with their intentions'. This reflects both the actus reus and the mens rea aspects of the statutory conspiracy. The fact the conspiracy is a continuing offence is important as the mere fact that Keith did not fulfill his part of the deal by giving Joe the funds does not mean that he is not guilty as the offence, it is committed as soon as there is an agreed course of action.
It is important that there is an awareness of the 'intended consequences' as 'liability maybe limited to these'. Therefore there would be some difficulty in arguing conspiracy in relation to the unforeseen death of Tracey at that stage. There would however be a substantial argument of conspiracy to murder Jennifer due to the aforementioned argument and that 'playing some part in' the conspiracy is seen as 'continuing to concur in the activity of another or failure to stop the unlawful activity' (Allen, 2007, p282). The second part of the scenario sees the consequences set in motion.
Joe's intent was to kill Jennifer however he inadvertently kills Tracey. 'Murder is the 'unlawful homicide committed with malice aforethought' (Allen, 2007). But if Joe did not have the required mens rea for murder it is possible to seek criminal liability for constructive manslaughter. Since Cunningham  intention to cause Grievous Bodily Harm as well as kill is a necessary element to commit murder as well as certainty (R v Woolin ). If the act and malice aforethought are not present then the party or parties cannot be criminally liable for murder but if there is a lesser degree of 'certainty' (R v Woollin).
In this scenario it suggests that although Joe's requisite mens rea element is not for Tracey, there is however a direct intention to kill Jennifer. The prosecution could argue that he would be criminally liable for the murder using the doctrine of transferred malice. Transferred malice in this scenario would be where you have the intention to kill or cause Grievous Bodily Harm to one person, but inadvertently kill another. It is argued that defendants in these circumstance have caused the 'actus reus' aspect of the 'murder and do intend to kill it is just the incorrect victim'.
This is also supported by case law in Latimer (1886) and subsequently by the Attorney General reference (No. 3 of 1994). The intent aspect of the crime is transferred from the intended to the actual victim therefore a mistake does not negate criminal liability for transferred malice. In this scenario the cause is not 'sufficiently far removed from the intended mode' and there is a direct intent which is transferred therefore causation and Woollin  do not need to be considered but suffice to say Joe initially created the danger and the death happened as a result of that danger.
Using this method the lesser degree of certainty required or the 'commission of the unlawful act' (Allen, Michael 2007) It could also be argued that at that stage Joe is also criminally liable for the attempted murder of Jennifer. This is now a statutory offence in the form of the Criminal Attempts Act 1981. It provides that a person who 'does an act which is more than merely preparatory to the commission of the offence' is 'guilty of attempting to commit the offence'.
Joe's actions in the scenario, of plotting Jennifer's' movements, being aware of her vehicle type, the time she would be driving past and being in possession of a bomb can clearly be seen as embarking upon a criminal enterprise but maybe as in R v Campbell could be seen as mere preparation but the pressing of the detonator itself could be sufficient and significant in proving 'more than mere preparatory'. (Allen Michael, 2007 p296)This was supported in R v Jones in which the act up until the event were considered preparatory but the pointing of the gun was considered 'sufficient evidence of an attempt' (Childs, Penny Criminal Law 2008)