Partial eating will get you in starvation mode. It will harm your body. Water is the best drink on the planet, even when you are not fasting. What will Senna tea achieve, it is diuretic and rob you from water. You do not want that on a fast that is healing and cleaning your body and mind.
I usually don’t recommend fasting for weight loss, you really have to ask yourself why you gained so much weight and how long did it take to get to this stage. Once you complete the fast you have to eat foods that heal you and help you continue the weight loss you desire. Western diet is full of processed foods and heavy proteins, both very toxic for your body.