Architecture

Yes, a BCA student can do B.Arch after completing graduation, no problem, but he has to get more than 80% marks in graduation level and complete graduation.

If you have a good GATE Score then it might become easier for you to get into a good institution and after that you will get Teaching Assistant Ship in that institution. In case, even you don't have good GATE score, then also there are institute that provides TA ship and many of the colleges have special scholarship for Obc and other reserved category students. Before taking admission, check whether the scholarship is available in that institution or not. For that, you can check their website or call them directly.

Yes, they do accept, but it depends on the which Bachelor's programme you choose, suppose you take BCT (Bachelors in Construction Technology) -
In that, you need JEE Paper - 2
But you can get more information from their CEPT website.

Delhi University has a fee structure of around INR 14, 000/year for B. Sc and M. Sc Programmes. You may also check the bulletin of Information released by the University each year with regards to changes, if any.

Civil engineering course is 4-years course
architecture course is 5-years UG course
Civil engineering is all about construction. Constructing building, roads, dams etc. Architecture course is about designing and making structures, techniques of designing.