Something like this: No nested if statements, fully expandable…
{% set number = states('sensor.front_door_db_alarm_level') %}
{% set sensor_codes = {'112':'Code Changed','113':'Code Duplicate','33':'Code Deleted'} %}
{% set sensor_code = states('sensor.front_door_db_alarm_type') %}
{% set code_status = sensor_codes[sensor_code] if sensor_code in sensor_codes else 'Code Error' %}
{% set user = 'Master' if number == '0' else 'User ' + number %}
{{ user }} {{ code_status }}
for your second question:
{% set number = states('sensor.front_door_db_alarm_level') %}
{% set entity_id = 'input_text.door_keypad_' + number + '_name' %}
{{ states(entity_id) }}