The BE rules appear in plain English on the DL website. All of OP's questions are answered.

With Basic Economy, you will not receive a seat assignment until after check-in or at the gate. Passengers traveling together, including families, may not be seated together. You will not be eligible for same-day changes or any other ticket changes or refunds after the Risk Free Cancellation Period. You will board in the last zone and not be eligible for paid or complimentary upgrades or preferred seats, even with Medallion® Status. Also, checked baggage fees will apply, even when traveling internationally.

There is no way to predict whether, if OP checks in at T-24, he will be assigned a seat at that time or closer to departure, perhaps at the gate. In any event, it is not a process in which he will participate, e.g., it is an assignment not a selection. Someone else on the same PNR may or may not be assigned to a seat close by but that is wholly dependent on availability. On a full flight, there may only be random middle seats. Thus, OP needs to plan on random middle seats.

A greater issue and this is something OP really needs to consider is that these tickets are wholly inflexible. They must be flown exactly as booked. No changes for a fee ---- either outright change or SDC. That can be a bad thing and suggests that if one is booking BE, one needs to hedge that savings with travel insurance which would cover a cancellation or change.