I am also looking at this paper, but I am not good at writng RNN too.

1: I think each string have an EOS represents the end of the string, the RNN will only make use of information before EOS

2: 1561 is calculated by: each position has 51 possibilities(US-Keyboard+Shift+Caps+Placeholder) for 1 insertion,and you have 30 positions(since length maximum =30). Also, you have 30 positions for deletion, which is 30, and last one for EOS, which is 1, then it comes to 1561. Assume that you have a password have length = 10, the class that related to position 11 to 30 just won't be activated(if it is predicting correctly).

3. For the Caps, Shift and Placeholder, you might refer to another prior work: https://ieeexplore.ieee.org/abstract/document/8835247?casa_token=Me_vgHsZvI4AAAAA:a2uw0Skz7iCjUbko8x5i_dVxouayxCpbZ5imdLRctc_j2D23Wvr6KmW7o1v53dJ8LcOhqSY