1. Is there a ban on donations from foreign interests to political parties?

Source: 

Section 5 (1) (b), Regulation of Election Expenditure Act, No. 3 of 2023

5. (1) A recognized political party, an independent group and a candidate at an election conducted under any law specified in section 2, shall not, directly or indirectly accept or receive a donation or contribution whether such donation or contribution is in cash or kind from-
(b) a foreign government, an international organization or a body corporate incorporated or registered outside Sri Lanka;
(c) a company incorporated in Sri Lanka under the Companies Act, No. 7 of 2007 or under any former written law relating to companies where the foreign shareholding in such company, either direct or
indirect, is fifty percent or above; ...
for the purpose of promoting or procuring the election of such recognized political party, independent group or candidate at such election.

https://www.parliament.lk/uploads/acts/gbills/english/6287.pdf 

There is a ban on donations from a foreign government, international organisation or a body corporate incorporated or registered outside Sri Lanka, to recognised political parties, independent groups and candidates.