بسم الله الرحمن الرحيم
Your saying ‘yes’ in response to the boys’ offer of nikah (like will you marry me) is a promise of nikah i.e. you said that you will marry them, then these two words are not ‘ijab w qubool’ (proper words exchanged for nikah) and no nikah will take place if such words were spoken whether such sentences were spoken in presence of some persons or not. But if you pronounced some other words which could be ‘ijab w qubool’ e.g. the boy said: ‘I marry you’ and you responded: ‘I accepted it’ in presence of Shariah witnesses i.e. two adult Muslims males, then you ask the question again quoting the exact sentence you spoke.
However, nikah will not take place if the boy and girl make promise of nikah as mentioned in the beginning of the question. And when no nikah took place, now the nikah solemnized by the parents will certainly be valid and binding.
(2) If ijab w qubool is done in the presence of Shariah witnesses the nikah will take place though the witnesses were not specified for the nikah but when they heard the ijab w qubool in the meeting of nikah the nikah become legally binding. But if the words exchanged were not meant for ijab w qubool rather they were made only for promise then nikah will not take place as it was mentioned in detail under answer 1.
Allah knows Best!
Darul Uloom Deoband