RE: how to teach the bot to answer after two (or more) non-understandable questions?

You should be able to use an "on repeat" for this.

default: I'm sorry but I don't understand that. Can you please reformulate that?
on repeat: I still don't understand. Please, let me know in which respect I can improve.

You can also use a "previous",

default: I'm sorry but I don't understand that. Can you please reformulate that?

default: I still don't understand. Please, let me know in which respect I can improve.
require previous: I'm sorry but I don't understand that. Can you please reformulate that?

You can also do more advanced things with "think", "condition" and some Self code,

condition: conversation.didNotUnderstand == null
default: I'm sorry but I don't understand that. Can you please reformulate that?
think: conversation.didNotUnderstand = 1;

condition: conversation.didNotUnderstand > 0
default: I still don't understand. Please, let me know in which respect I can improve.
think: conversation.didNotUnderstand = conversation.didNotUnderstand + 1;