4. Is there a ban on corporate donations to candidates?

Source: Law on Political Parties, 1983 (amended 2018)

Presidential candidates shall not receive donations from legal persons.
Source: Article 14(1), Law on Presidential Election, 2012

Persons employed as civil servant or officer or in the State administration, annexed budget administrations, special provincial administrations, municipalities and agencies and organizations affiliated thereto, state economic enterprises and the agencies and partnerships established by them, and in other public organisations, cannot distribute handouts.
Source: Article 62(2), Law on Basic Provisions of Elections and Voter Registers, 1961 (amended 2018)

There are no specific regulations regarding donations to candidates except for presidential ones. There is also limitation on distribution of handouts (see details "Source" section".