Your fiancé has to take permission from her guardian. As you mentioned that her father has passed away( may Allah grant him place in Jannah)
Now her mother is guardian her permission is must.
Yes after the muhtaj you can do the permanent marriage. But for example if you are doing Muta for 15 months and if month or days left before your permanent marriage then in this situation you have to gift her the remaining period then you can do permanent marriage.
As per the shariah it is not Haram before arbaeen. But only few days left if in respect of aza if you wait for few days it will be better.