Gstr3b urgent

You can add missed input of Nov month with Dec Input in 4 (A) (5) and proceed.

ITC of any month missed in an FY can be taken before September of next FY.

ITC cannot be taken beyond the month of September of the following FY to which invoice pertains or date of filing of an annual return, whichever is earlier.

Yes you can claim the ITC in the GSTR 3B of the next month. If the ITC was related to reverse charge then report the same as:

Point 4: Eligible ITC

a. under reverse Charge   - if related to reverse charge

b. All other ITC - other than reverse charge and import