[Python] How to change texts in columns and then combine 1st row with the column headers?

I have the following dataframe:

data={'a1':['X1',2,3,4,5],'Unnamed: 02':['Y1',5,6,7,8],'b1':['X2',5,3,7,9],'Unnamed: 05':['Y2',5,8,9,3],'c1':['X3',4,5,7,5],'Unnamed: 07':['Y3',5,8,9,3],'d1':['P',2,4,5,7],'Unnamed: 09':['M',8,4,6,7]}
df=pd.DataFrame(data)
df.columns=df.columns.to_series().mask(lambda x: x.str.startswith('Unnamed')).ffill()
df




There are a few things which I would like to do:

1. Change the rows containing (X1, X2 & X3) into just 'X (vice versa for Y1,Y2,Y3 into 'Y)
2. Combine the existing column header with the row containing X,Y,P,M

The outcome should look like:

1. Change the rows containing (X1, X2 & X3) into just 'X (vice versa for Y1,Y2,Y3 into 'Y)

1. Combine the existing column header with the row containing X,Y,P,M - Also note that the 'P' and 'M' completely replaces 'd1' respectively.

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