The dining plans have ALWAYS been tied to the NIGHTS you stay. They have to pick one or the other and since it requires a resort stay in order to even get the plan, nights works best. Otherwise, people could do a one night stay, but buy the plan for a whole week and Disney loses money.
As to using your TS credits for QS, you were told partially wrong information. You CAN use your TS credits for ONE QS meal. It is a one-for-one exchange. They won't give you 2 QS meals for 1 TS credit. So if you want to get a QS meal instead, you can. All your other QS credits will need to be used before this can happen, though.