To keep leads after converting to opportunity select that lead before converting to oppurtunity and duplicate it using Action button and save it.

for second question i think you can do it by managing filters and group by.

Thank you

I have know the answer for 2nd question: remove manager from portal group,  the default filter will not apply to manager any more, then manager can see other users and assign leads to them.

Still dont know how to deal with first question.

And why  there is  -1 label for this question.