DP (Depository Participant)
Demat Account is similar to a locker where your shares are stored in electronic form.
When you have a trading account it is not necessary to have a Demat account also....with the same broker
Disadvantages for a frequent buyer/seller of shares are many considering each settlement you will need to issue the instruction slip within the stipulated time to your DP.
There are many more...but the one outlined above will suffice for the moment.
a) Broker cannot force client to have a demat account with them along with the Trading Account.
b) POA is not mandatory.... infact it is voluntary
If they are forcing you to do something against SEBI regulation then I suggest you take a screenshot of pages on their website and lodge a complain to SEBI.