As long as Salary is credited into any of your DBS/POSB bank accounts under your name, can already. However, only the money in the Multiplier account earns the interest. Hence it is common practice to put more funds in your Multiplier account, and leave enough for normal spending in your normal DBS/POSB account. So, yes, to "enjoy the higher interest", put in more funds into Multiplier account.
Yes, you can freely transfer funds between Multiplier account to your other DBS/POSB Accounts.
Yes, you can withdraw from your Multiplier account. You can either 1) Have a basic ATM or DBS Debit Card card, and link it to both your normal DBS/POSB account and Multiplier account for ATM cash deposit/withdrawal use or 2) Just apply one ATM / Debit card for each account ATM use.