Question: What was the deal with the woman in the bank? Why did she get a bag of money at the end of the movie?
Answer: Thanks to her, the group was able to measure the vault in which all the money from the race would be stored, as to know distances and the amount of garbage bags to bring and to ensure the shortest time possible. So they gave her the money for aiding them without her knowing.
The cake they brought her attracted the painted roaches. Different colored roaches were released at different tubes. The pink ones that showed up in the bank told them which tube led to the vault.
Question: What was the purpose of Channing Tatum preventing the gate from opening?
Answer: Jamming the gate by blocking the sensor delayed Joe Bang's brothers long enough for Mellie to come and collect some bags full of cash. Jimmy specifically did not trust them to know his entire plan. This part of the ruse allowed them to steal a separate stash of money so they could pretend to return the stolen money, whilst secretly keeping some to distribute later.
Question: At the end, how does Joe know where to dig?
Chosen answer: Earlier in the film, it's mentioned that he had some money buried in a special spot in his yard, but that it had been dug up. When he saw the shovel at the end (which indicates that something was buried that he should did up), he realised that the new money stash had been buried in the same spot as his old stash. They basically gave him the clue, and he put two-and-two together on his own.
Answer: In addition to the already provided answer, they reimbursed her for the damage to her car.