Part 2.
— depending on what our 18-yo son chooses to do (college out of state? live with Mom or Dad? etc), the HOH filing status is on the table
Yes, but there are a lot of moving parts to consider. A child living away from home while attending college is generally considered to live "at home" because college is a temporary absence. In the case of divorce or separation, the child is treated as living with the parent he would have lived with if not away at college. (This can be determined by facts and circumstances, such as where are his friends, his doctor, his other significant relationships, and all his personal items that he can't take to college, and where does he return for summer vacation.)
If a child definitively moves out of their parent's home and establishes their own separate residence, with no intent to return to the parent's home except for short visits (no longer use the parents home as a residence), then the child does not live with either parent, and can't be claimed any more as a dependent or used to qualify for HOH. Although even here, what counts is where the child lived more than half the year, so even if the child definitively moves out in September, they still "lived with" their parent for more than half that year and the parent can generally claim them as a dependent.
There is also a financial test. If the child provides more than half their own support, they are not a dependent or qualifying person for HOH. Support the child provides themself include money they earn, their own savings that they spend, and student loans in their name. Support provided by the parents includes any cash payments for tuition, room or board, other cash assistance, the value of maintaining a residence in the parent's home, and any tuition loans taken out by the parents. It is possible, depending on job and tuition, for a college student to provide more than half their own support and no longer be a dependent of either parent.
