Weak entity having n instances as the 'one'
You can combine B and C to one new key that is used in A and also have a key from B directly, it is no problem as you have two keys.
If you don't add something new to the key you would have the same key, but referring to different entities A and C.
Ok as long as you don't try to 3NF or higher, it is at least possible but redundant.

so C being weak can take n instances of A which is also weak and attached to the same owner?

Your relationship T is wrong. A weak relationship is always 1 to n or 1 to 1, never n:m. The essential part is that the weak entity has to know which key it relates to, see http://infolab.stanford.edu/~ullman/fcdb/slides/slides2.pdf, so if it where a 1:n , it is possible.