1. What rights for individuals are implied into Article 3; what character (absolute or qualified) do these rights have?
The right to stand in an election for the legislature (i.e. not to be prevented by law), the right take a seat in the legislature if elected, and the right to vote. These are all inferred from or implied by the text of Article 3 and can be subject to reasonable restrictions imposed by the law.
2. What is the proper role of the ECtHR in considering exceptions to the individual rights in Article 3?
It accepts that states have a wide margin of appreciation. Its job is as a reviewing court to ensure that any restrictions do not undermine the essence of the rights to vote, etc. and do not thwart the will of the people.
3. What kind of voting system does Article 3 require?
Article 3 does not require any particular voting system. Whatever system is chosen it must apply equally to all.
4. Does Article 3 require a fully democratic political system with all parts elected?
No, though, clearly, important parts of the legislature must be elected.