: Problem 2: Each half of the cycle lasts 1000 years, not
: each full cycle.
Nope! The Myth II Epilogue says that the first Leveller was killed in the Second Era... a thousand years later he was killed again... again in the Fourth Era he was killed again... in other words, the Dark cycle *ends* every thousand years, meaning that it has to begin somewhere between there, and it makes sense that it would begin right between the endings, 500 years before or after the last or next one ends. In other words:
0000 - Cycle starts, First Era begins
0500 - Some hero becomes Leveller
1000 - Leveller dies, Second Era begins.
1500 - Great Hero becomes Leveller.
2000 - Leveller dies, Third Era begins.
2500 - Great Hero becomes Leveller.
3000 - Leveller dies, Fourth Era begins.
4500 - Great Hero becomes Leveller.
5000 - Leveller dies, Fifth Era begins.
5500 - Tireces becomes Moagim.
6000 - Moagim dies, Sixth Era begins.
6500 - Connacht becomes Balor.
7000 - Alric kills Balor, Seventh Era begins.
See?