We invited more people than came.
 Fred reads more books than Susan reads.
Yes:  and  are both correct. "Than" is a preposition, and "came" and "reads" are comparative clauses functioning as complement of "than".
Comparative clauses are stucturally reduced in some ways relative to the structure of main clauses. In full, the clause in 1. would be *"We invited more people than people came". But no one would say that, in fact it's ungrammatical; instead the comparative clause is reduced to a single element, the verb "came".
Similarly, in  the comparative clause is reduced to just the verb "reads". In full it would be "Fred reads more books than Susan reads books", again ungrammatical.
a. We invited more people than they came.
b. Fred reads more books than Susan does.
a. is a doubly ungrammatical attempt to fill out the reduction in . See my comments above.
b. is fine and a grammatically correct alternant to . "Does" is also a reduced comparative clause understood as "Fred reads more books than Susan reads books".