There are no limits on the size of the bonuses or on how many can be received.
If your account currency is not in USD, then all calculations will be performed in USD and then converted to your account currency using the platform rate at midnight UTC.
No, the Total Bonus will remain inside the account even if you withdraw all Balance from the Account. The amount of the Total Bonus will remain inside the account, however, this will affect the Active Bonus which in this case would be zero. If you re-deposit, than you can reactivate your Bonus.
In this case, your entire Bonus will be accessible, meaning both Total Bonus and Active Bonus will be of equal value.
cTrader Bonuses do not affect Equity negatively in any way. If the Bonus is converted into the Balance, Equity will remain the same or increase. An increase in Equity will occur if an accounts Bonus is greater than Balance.