Can I buy insurance in the ER?

It's very unlikely that you will be able to obtain instant health insurance coverage in (or on the way to) the ER. Even though it prohibits denying coverage based on pre-existing conditions, the ACA doesn't require that coverage be provided immediately and has some provisions to reduce adverse selection. One way it does that is through open enrollment periods. Unless there is a qualifying event (job loss, divorce, etc), in most cases individuals won't be able to obtain coverage outside of the annual open enrollment periods (the first period will last 6 months but subsequent ones will be much shorter) and instead have to wait until the next open enrollment period. Even if you needed emergency care during the open enrollment period and managed to send in an application and premium payment in (or on the way to) the ER, it's unlikely that the effective date of coverage would be the same day or even the next one. In a best case scenario, it might take 48 hours or even several business days.

Correct, that insurance companies cannot deny you coverage for a pre-existing condition, but they will not pay for any bills you rack up before you bought your policy. There also will be a processing time before the policy is bound (in effect) and will pay anything. Insurance is not retroactive like that. Should this happen, the best you could do would be emergency Medicaid. Hospitals also normally have payment plans to pay off debt as well.

Hospitals are required by law to treat anyone who comes to the Emergency Room. You will have the penalty for not having insurance and all the bills associated with your uncovered services. You can apply for insurance and will be covered from the day your premium is paid and the policy is in force which will not be back dated to your Emergency Room date

Clearly by the law you can now buy insurance but there will be many constraints over the top because insurance company are also very clever and they won't let any loose money slip out of their accounts.