1. Essentially, once you sign a contract for employment, then you can be sued for breach of contract if you break it by not turning up for work as agreed. The measure of damage will be the amount the employer has lost by interviewing people and to get an employee and spending money in that regard. Essentially, you are liable for their loss in hiring you. However, be aware that in practice, many employers do not go to the length of suing employees who fail to work after signing a contract of employment. This is purely for the fact that an employer wants to be able to hire someone else, so suing people for not fulfilling their employment contracts is bad publicity. So many don't go to those lengths unless there is the hiring of executive search or some large amount of expenses involved.
2. Essentially if you sign a new contract it supercedes the original agreement. It does not make the original agreement null and void. Until both parties agree the original contract or agreement is set aside, it continues in force and you can still be sued on the basis of it.
3. Potentially, yes. But the employer is under a duty to mitigate their loss. In practice, this means that they have to get the next available candidate as soon as possible. So, in practice it is not possible to argue that more than a week went bye without getting someone from the disappointed candidates.
4. Legally speaking, you have to give the notice in the contract. So this would essentially mean giving notice on the first day of work.