1. A 2930F VSF stack can have ###EDIT### 8 members as max. But design recommendations say stick on max 4. In early firmware the max. was 4 members, but in later firmware its changed to 8 members.
2. I never use the GUI, learn CLI and use Airwave for management. For password complexity yes GUI/Rest must be disabled.
3. I would recommend to use a TACACS+ server so you can use different AD accounts to manage the switches. But you need off course a TACACS+ server like Aruba ClearPass to make this happen.
Without TACACS+ (or RADIUS);
My standard approach (smaller environments) is not using password complexity feature but use a strong manager password (or create your own account name) and make sure that switch management is only reachable from the management vlan for administrators. You can also set from what source ip pool management is reachable. For bigger environments with many administrators use TACACS+ with Aruba ClearPass.
For more information about the password complexity feature and configuration examples see this document: https://h20628.www2.hp.com/km-ext/kmcsdirect/emr_na-a00038763en_us-3.pdf