Miswaak while fasting;qadha fasts

Asalamu- alaikum..i would like to know whether using a miswaak is permissable whilst fasting and is it a sunnah?
Also, if you dont make an intention in the night to keep a qadha fast, and you dont wake up for sehri, but when you wake up in the morning you make the intention and keep the fast is that fast accepted?
can you make the intention of keeping both qadha and shawwal fast? if i’ve been doing this what niyyah would have been accepted?