Let me explain the answer for the First question:

Write one function for Combo1

n=datatable.value("selectStr",dtlocalsheet)'this statement 
will send parameter to the first combo for selection of 
a,b,c,d etc

Public function combo1Selection(n)
  Browser().page()....select n
  'put standard checkpoint on Second combo and select table
  i=Browser().page()....webtable().getROproperty("value")
  if i="expected" then
     'write statements
  else
    'write statemets
  end if
end fucntion



*****Random testing of 500 test cases in qtp*****

Hi u can use excel sheet for testing this....
u can use parameter in the excel as "Run" and in the "Run" 
field just specify as "Yes" whichever testcases u want to 
run.
this has big procedure....if u want ..just send mail to 
djagan2000@yahoo.com