You can find "Jumping" numbers across AX and it's ok. As I said you can use continuous number seq for PO in that case AX will reuse all number so you won't have any gaps. However, it has performance impact as well and general recommendation from MS is to use them only if there is legal requirements. I have no idea if you have it in Singapore so it's up to you. You can read more here technet.microsoft.com/.../hh209457.aspx.
PO is a source document so you can create one by mistake and delete it immediately, it should not be a problem. I have never heard about some one looking at PO's or any requirements to store PO, only Invoices. Don't forget that PO could be changed, that's why you have history on it and you can post confirmation as well.
As far as I know there is no audit for number seq in AX, so probably data base log is only option.