Okay my 2 cents take on this.
1. TS has property loans and joint account with the Bank A. The property loan is joint tenancy and joint saving accounts 6 digits which is allows any party to withdraw. TS did not clarify how much is 6 digits, is it 500k, 600k, 800k cash in the saving account.
2. TS did not indicate the number of guarantors to the 800k loans, it should be possible for any number of the guarantor to the bank's satisfaction.
3. Putting aside whether the hdb can be a collateral to the business loan of 800k, which obviously no bank can allow to use hdb as collateral. Here, all i can say is Bank A definitely has the unilateral right to rescind or cancel a property loan, be it hdb or condo. Bank A dont really need to act in anyone's interest, it definitely can suka suka early terminate, in addition, all T&S conditions definitely has it that in bankruptcy or any other, the bank can early terminate.
4. Hence, what is very clear is that, Bank A has already (or going to) terminated the property loan even if TS did not default on the loan installment, the crux is that if bank A has already terminated loan, nobody, not even hdb, MAS, Member of parliament or govt can stop the bank from cancelling the loan nor force the bank reinstate back the bank loan. That's not important, whats important is that in a normal scenario, the borrower needs to refinance from another bank or if not TS must pay up the liability upon the notice period, hence the 400k.
5. What i do not know is this, is TS able to refinance from another bank?? Not very sure if i am right but i am afraid TS is probably unable to refinance because simply put that Bank A will not allow the refinancing as it has lien over the property due to TS's mother acting as the guarantor. Bank A has 50% share of the HDB and definitely has 100% ownership of the joint saving account. Bank A will not allow the other party to take over the HDB without the share of the HDB flat value owner by TS's mother. The law do not allow the bank A to sell the property without consent with all the owners agreement, however, the Bank A can force sell if TS do not pay the 400k. Hence, the question here is only whether can TS pay up the 400k, is he able to seek refinancing.
6. TS should seek HDB's counsel's help immediately or go to supreme courts or places where there is free lawyer service. (its 30mins for the free law advice)
7. Why is the bank A able to make HDB an implied collateral through such a structured package deal, i think it is a question for the law makers.