Generally, no. In most cases, either state law or the terms of the account provide that you usually cannot remove a person from a joint checking account without that person's consent, though some banks may offer accounts where they explicitly allow this type of removal.
The best way to find out how exactly you can change a joint account to a single is to call your bank and ask or just go into a branch and talk to someone in person. Then, you can open a new single account if you want to.
Investments and savings will generally form part of your financial settlement on divorce or dissolution. Dividing them should be relatively straightforward if you can negotiate with each other. But you may need to value them and pay tax or charges if you sell or transfer them or cash them in.
No, when someone dies owing a debt, the debt does not go away. Generally, the deceased person's estate is responsible for paying any unpaid debts. The estate's finances are handled by the personal representative, executor, or administrator.
When you get a divorce, you are still responsible for any debt in your name. Most states follow "common law," which means that a court will hold you responsible for any credit card debt that is solely in your name, and will hold you jointly liable for credit card debt that is in both your name and your spouse's name.
Even if your spouse opens up a line of credit in their name only, you could still be liable for that debt. Creditors can go after a couple's joint assets to pay an individual's debt. In that case, the creditor can only go after the person responsible for the debt.
Joint debts. In the event that a relative co-signed on a credit card debt or loan, they will be liable to pay it off even after death of the co-signee.
The Wall Street Journal explains that, unless you refinance that debt together, your credit histories remain separate and you don't take on their debt: In general though, no, you're not legally responsible for your new spouse's old debt.