I don't see why you would need an accountant unless you want that level of handholding, there's nothing unusual about your situation and if you follow the interview, answer all the questions, and don't skip any steps, you should be just fine.
1. Your 24 year old child (over 23 as of December 31, 2017) can't be your dependent if she earned more than $4050 of taxable income, even though she is on your ACA plan.
2. Your 22 year old child (23 years old or less on December 31, 2017) can probably be a dependent of either you or your ex. He is your dependent if he does not earn more than half his own support and if he lives with you more than half the year. College is considered a temporary absence and your child is considered to "live with you" while away at college if they would have lived with you if not for college.
Because you separated in May, your son might or might not have spent enough time with your ex to be their dependent as well. Both you and your ex get "lived with" credit up until the separation, so lets estimate 140 days. If your son lived with your ex for another 43 days, then he "lived with" your ex for "more than half the year" and your ex can claim him as a dependent. Your son of course "lived with" you for more than half the year as well, and probably for more days than your ex, taking into account college and the "temporary absence" rule. If you and your ex both claim your son as a dependent, the tiebreaker is which parent did the child live with the greater number of days. So while there would be an investigation, the dependent exemption should ultimately be awarded to you. (Hopefully your ex will not contest this to begin with.
Because your son is under 24, the support rule is not a specific dollar figure but is "provided less than half his own support." Support your son provides himself includes money he earns and tuition he pays with loans that are in his name. Support you provide includes tuition that you pay (from money or from loans in your name), money you provide for books, living expenses, transportation, etc. Your support also includes a 1/3 share of your home rental value and utilities (since you are providing him a place to live) and 1/3 of your ACA premium. (Grants and scholarships are not support factors on either side in this case.)
So if he does not provide "more than half" his own support, and if he "lives with you" more than half the year, he can be your dependent.
3. If your son is your dependent then you can file as head of household instead of single, which is a bit more favorable to you.
4. Any property or assets that are divided as part of the divorce are not taxable to you or tax deductible by your ex.
5. If your ex is required to make ongoing support payments (alimony, spousal maintenance) those payments are tax deductible by him and taxable income to you (unless your divorce decree specifies that ongoing maintenance payments are not alimony for tax purposes).
6. Child support payments (such as, your ex is required to pay child support or college expenses for your children, or is required to pay part of the ACA premium for your children) are not alimony and are not deductible by him or taxable to you. (But note that if, for example, he was required to pay half your family's ACA premium, the part that goes towards the kids insurance is not alimony but the part that goes toward your insurance is alimony.)
