Concatenate string in .csv after x commas using shell/bash - bash

Question

I have several .csv files containing data. The data vendor created the files indicating the years once in the first line with missing values in between, variables names in the second. Data follows in the third to the Xth line.
"year 1", , , "year 2", , ,"year 2", , ,
"Var1", "Var2", "Var3", "Var1", "Var2", "Var3", "Var1", "Var2", "Var3"
"ABC" , 1234 , 4567 , "DEF" , 789 , "ABC" , 1234 , 4567 , "DEF"
I am new to shell programming but it shouldn't be too complicated writing a script that outputs the following
"Var1_year1", "Var2_year1", "Var3_year1", "Var1_year2", "Var2_year2", "Var3_year2", "Var1_year3", "Var2_year3", "Var3_year3"
"ABC" , 1234 , 4567 , "DEF" , 789 , "ABC" , 1234 , 4567 , "DEF"
Some thing like
#!/bin/bash
FILES=/Users/pathTo.csvfiles/*.csv
for f in $FILES
do
echo "Processing $f file..."
# 1. Replace the second line with 'Varname_YearX' where YearX comes from the first line
cat ????
# 2. Delete first line
sed -i '' 1d $f
done
echo "Processing complete."
Update: The .csv files vary in their amount of lines. Only the first two lines need to be edited, the following lines are data.

Answer 1

If you want to merge the first and the second line of each CSV, try this.
# No point in using a variable for the wildcard
for f in /Users/pathTo.csvfiles/*.csv
do
awk -F , 'NR==1 { # Collect first line
# Squash quotes
gsub(/"/, "")
for(i=1;i<=NF;++i)
y[i] = $i || y[i-1]
next # Do not fall through to print
}
NR==2 { # Combine collected with current
gsub(/"/, "")
for(i=1;i<=NF;++i)
$i = y[i] "_" $i
}
# Print everything (except first)
1' "$f" > "$f.tmp"
mv "$f.tmp" "$f"
done
The first loop simply copies the previous field's value to y[i] if the i:th field is empty.

Answer 2

Ugly code using csvtool, various standard tools, and bash:
i=file.csv
paste -d_ <(head -2 $i | tail -1 | csvtool transpose -) \
<(head -1 $i | csvtool transpose - |
sed '$d;s/ //;/^$/{g;b};h') |
csvtool transpose - | sed 's/[^,]*/"&"/g' | cat - <(tail +3 $i)
Output:
"Var1_year1","Var2_year1","Var3_year1","Var1_year2","Var2_year2","Var3_year2","Var1_year2","Var2_year2","Var3_year2"
"ABC" , 1234 , 4567 , "DEF" , 789 , "ABC" , 1234 , 4567 , "DEF"