IS IT POSSIBLE TO INSTITUTE A NATIONAL PHASE ENTRY IN JAPAN AFTER 30 MONTH DUE DATE?

NO.
The Japanese Patent Law is very restrictive to the time limit. The demand paper must be filed within the 30 month due date(*) though the translation can be submitted later.

WHAT NUMBER OF CLAIMS IS SUBJECTED TO THE FILING SURCHARGE (EXTRA CLAIM FEE)?

NONE.
The official filing fee is flat and independent from the number of claims in Japan, while the substantive examination fee (*) and the issue fee are dependent on the number of claims.

*: A request for examination must be filed within 3 years from the international filing date, together with payment of substantive examination fee.

IS IT REQUIRED TO PAY A MAINTENANCE FEE EVERY YEAR?

NO.
In Japan, no maintenance fee for making an application pending before the Japan Patent Office is required.