For a long time many scientists suspected that koalas were so lethargic because the compounds in eucalyptus leaves kept the cute little animals in a drugged­out state. But more recent research has shown that the leaves are simply so low in nutrients that koalas have almost no energy.

Can i use had suspected or had been suspecting instead of suspected? To me, it seems possible because For a long time, action of the verb was maintained for a certain duration of time.

Two of them have same meaning in the context above?

For a long time many scientists suspected that koalas were so lethargic because the compounds in eucalyptus leaves kept the cute little animals in a drugged­out state. But more recent research has shown that the leaves are simply so low in nutrients that koalas have almost no energy.

Can I use had suspected or had been suspecting instead of suspected? To me, it seems possible because "For a long time", the action of the verb was maintained for a certain duration of time.

Do the two of them have same meaning in the context above?