You really need to speak to a solicitor to get all these answers as, whilst they are not complicated, they require a lengthy explanation. I’ll try to be brief and answer your queries:
If the probation service has been asked to prepare a pre-sentence report the Magistrates will take heed of its recommendations.
The length of time to get to a trial in the Magistrates’ Court varies from area to area, but you’re probably looking at about three months.
He is unlikely to go to prison for common assault and criminal damage unless the offence is particularly serious. As far as conviction goes, if he pleads guilty he accepts what the prosecution is saying and no evidence will be needed in court. If the matter goes to trial the prosecution will almost certainly require your mother to attend court and give evidence (and they will issue a witness summons if necessary). She will be questioned on both her statements and may be declared a “hostile witness”. This means she can be cross-examined by the prosecutor and asked to explain the substantial differences between her two statements.