Printing the compared result under last column header - awk

Question

I'm doing a comparision of 2 files file1,file2 using first column in file1 to first column in file2 and retriving corresponding value from 7 th column .
awk -F, 'FNR==NR{a[$1]=$7;next} {print (($1 in a) ? $0","a[$1] : $0",NA");}' file2.txt file1.txt > tmp && mv tmp file1.txt
also on next day it will compare and append the result .
cat file1.txt
N1,N2,N3,N4,N5,N6,D1,D2,D3,D4,D5,D6,D7,D8,D9,D10
XX,ZZ,XC,EE,RR,BB,OK,OK,OK,OK,OK,OK,OK,OK
XC,CF,FG,RG,GH,GH,NA,NA,NA,NA,NA,NA,NA,NA,NA
DM,DF,GR,TH,EW,BB
cat file2.txt
cat file2.txt
DF,GH,MH,FR,FG,GH,NA
XX,ZZ,XC,EE,RR,BB,OK
awk -F, 'FNR==NR{a[$1]=$7;next} {print (($1 in a) ? $0","a[$1] : $0",NA");}' file2.txt file1.txt > tmp && mv tmp file1.txt
mv: overwrite `file1.txt'? y
cat file1.txt
N1,N2,N3,N4,N5,N6,D1,D2,D3,D4,D5,D6,D7,D8,D9,D10,NA ---> Header
XX,ZZ,XC,EE,RR,BB,OK,OK,OK,OK,OK,OK,OK,OK,OK,OK
XC,CF,FG,RG,GH,GH,NA,NA,NA,NA,NA,NA,NA,NA,NA,NA
DM,DF,GR,TH,EW,BB,NA
after adding new row
DM,DF,GR,TH,EW
problem is it is comparing and printing result for header also and result is printed
under header D1 instead of D10 for newly inserted row in file1
How can we print like this, compare should exclude header and result under last column header
N1,N2,N3,N4,N5,N6,D1,D2,D3,D4,D5,D6,D7,D8,D9,D10
XX,ZZ,XC,EE,RR,BB,OK,OK,OK,OK,OK,OK,OK,OK,OK,OK
XC,CF,FG,RG,GH,GH,NA,NA,NA,NA,NA,NA,NA,NA,NA,NA
DM,DF,GR,TH,EW,BB ,NA

Answer 1

To avoid having header updated, change awk's expression to the following:
'FNR==NR{a[$1]=$7;next} FNR==1{print $0; next} {print (($1 in a) ? $0","a[$1] : $0",NA");}'
In this case 1st line of the file1.txt will be printed as is, without any changes.
But don't you also need to have new day (like "D10" in the example) be added to the header on each run? Or you do it elsewhere?
As to the 2nd question (printing new value at the same position in the string for the shorter line as for the longer line), you should further modify awk:
'FNR==NR{a[$1]=$7;next} FNR==1{print $0; len=length($0); next} {printf $0; cont=(($1 in a) ? ","a[$1] : ",NA"); for (i=length($0)+1;i<=len-length(cont);i++) printf " " ; print cont;}'