It is true you can overdose on bananas, but it's highly unlikely. According to BBC.com, you can overdose on bananas, but it's highly unlikely a healthy person can. It would take 400 bananas to overdose. The article states, "Adults should consume about 3,500mg of potassium per day, according to the UK's National Health Service. The average banana, weighing 125g, contains 450mg of potassium, meaning a healthy person can consume at least seven-and-half bananas before reaching the recommended level." To americanize, that's 0.1234589 ounces per day, The average banana is 4.40925 ounces, containing 0.015873283 ounces of potassium. To answer your second question, It's generally unclear, however, if you do consume too much potassium on a daily you can develop a disease called hyperkalemia, which can lead to long-term effects like abdominal pain, diarrhea, nausea, vomiting, chest pain, irregular, fast, or fluttering heartbeat limbs that are weak or numb. All in all, you can die from bananas. However, It is difficult to and there are long-term effects, but it's again hard to tell or to develop.
sources:
https://www.webmd.com/a-to-z-guides/hyperkalemia-potassium-importance
https://www.bbc.com/news/magazine-34225517#:~:text=But%20for%20a%20healthy%20person,%2C%20very%20good%20for%20you.%22
https://www.kidney.org/kidney-topics/hyperkalemia-high-potassium