Aggregating Values That Match Aggregate Result

The actual schema I'm working against is proprietary and also adds more complication to the problem I'm trying to solve. So to solve this problem, I created a mock schema that is hopefully representative. See below for the mock schema, test data, my initial attempts at the query and the expected results.

-- greatly simplified schema that makes as much sense as the real schema
CREATE TABLE main (keyvalue INT NOT NULL PRIMARY KEY, otherdata VARCHAR(10));
CREATE TABLE dates (datekeyvalue INT NOT NULL, keyvalue INT NOT NULL, datevalue DATE NULL, PRIMARY KEY(datekeyvalue, keyvalue));
CREATE TABLE payments (datekeyvalue INT NOT NULL, keyvalue INT NOT NULL, paymentvalue INT NULL, PRIMARY KEY(datekeyvalue, keyvalue));

[Code] ....

Desired results:

SELECT 1 AS keyvalue, 'first row' AS otherdata, '2015-09-25' AS nextdate, 30 AS next_payment
UNION ALL
SELECT 2, 'second row', '2015-10-11', 150
UNION ALL
SELECT 3, 'third row', NULL, NULL

I know I'm doing something wrong in the last query and I believe another sub-query is needed?

Let me answer a few questions in advance:

Q: This schema looks horrible!
A: You don't know the half of it. I cleaned it up considerably for this question.

Q: Why is this schema designed like this?
A: Because it's a 3rd-party mainframe file dump being passed off as a relational database. And, no, I can't change it.

Q: I hope this isn't a frequently-run query against a large, high-activity database in which performance is mission-critical.
A: Yes, it is, and I left out the part where both the date and the amount are actually characters and have to pass through TRY_CONVERT (because I know how to do that part).