the problem is your trying to achieve 2 different things, both weight gain and weight loss, because the reason you are lacking in abdominal definition is because you have alittle fat there, stretching and yoga may help with tighting the skin it may not.
i have a freind who lost alot of weight she used exercises which toned loose skin ill ask her what she did.
there some myths which you first need to understand
1. doing sit ups will not give u a toned stomach
2 losing weight fast will get you abs
you need to lose fat generally before it starts thinning around your tummy, so cardio is the way , coupled with strength training and a good diet
key points take cardio very gently, why because when u feel the burn it means you have a lactic acid build up and lactic acid hinders the break down of fat molecules.
and fat is not released for burning for upto at least 20 mins, so start counting after you have been doing your cardio for 20 mins,
muscle burns more calories then fat! see your body to exist function to move needs energy , " calories" and as you sit and rest your body is still using energy " calories"
so you need to maintain more muscle as this will increase your metabolic rate, hence the importance of resistence training
lastly should your goal be geting to 9 stone? or should it be stability fitness good health and looking good, i hope this has helped i can go into more detail but ill write forever and make you sick.
heres the dilema u face
for weight loss u need to be less calories then u r using daily
and for weight gain u need to do the complete oposite
my advice look to gain muscle, you will lose your tummy in the process , gain weight too, but to do that you need to eat more not less;)
Help me , help you