Smells like you have yourself what i'd call a "client from hell".
Kwanza kwa nini hakuna uwezo wa kuipata kutoka kwa designer wa mwanzo? anawadai? Kama wanadaiwa waambie wamlipe na wewe pia hawatakulipa usipoangalia, you need to contact the original designer and become his/her friend.
Pia unaposema dau lao ni dogo inaonesha between you two(you and your clients) either both of you are not serious or one of you is not.
Website sio nyanya na wala sio one-night-stand, Website ni kama ndoa, its a 'til-death-do-us-apart' thing, inahitaji commitment from you and your clients. Clients wanaotaka vitu vya bei rahisi huwa ni wasumbufu kupita maelezo so you have been warned, lipwa advance (at least 50% ) kabla hata haujaanza kazi(this will force them to commit to the project).
By the way there's no way you can do it without the original designer's approval, otherwise you can use a different domain and forget everything about the current site, either way, be careful with these clients.