the only difference is the middle letter. ICOs are scamming people by calling themselves initial "coin" offering and IEOs are scamming people by doing the same exact thing while calling themselves initial "exchange" offering.
the end result is exactly the same with different speeds at which they dump and exit. as IEOs are already supported by and listed on an exchange they can dump and run away faster.
You got it absolutely correct. The only difference is that C and E in between. Otherwise, they are both the same bananas. They are actually both coin offerings, the other one offered to the open public from the project itself while the other one is offered through the exchanges. There are tinier differences such as ICO coins might not be listed at all while IEO coins are going to be listed immediately right after the offering, ICOs are offered in an absolutely free market where a scam is offered alongside a much lesser scam while IEOs are somehow screened by a third party which is the exchange, and so forth. But all these differences could really be insignificant.