Welcome to AnswerBank. You only need to ask the question once rather than starting 3 threads.
Breaking your question down:
1. You'll have to manually calculate the employees wage for the balance of the month, after coming off the weekly wage, although I assume that she was only getting SMP until she returned to work. When you do the payroll run for monthly paid employees, remember to highlight her name with the others, unless you are running Sage with 2 companies (1 for weekly paid and 1 for monthly paid, in which case you will have to set her up again in your monthly paid company), or have you set your employees into groups with different pay periods, in which case you will have to change her group.
2. Tax code change - just alter her tax code field in Employee Details, Sage will calculate the tax correctly at the next pay run.
3. Any SMP will be calculated on her average earnings in the period before she started Maternity Leave, therefore will not be affected by the increased salary.
Sage Payroll is very simple to use, and also has a very extensive Help facility.
Good luck