I do not know if you plan on going into the parks at all, but there's always going to IoA while your mother enjoys HHN at the beginning until IoA closes. Of course, it's not open extremely late, but you'd have a couple of more hours there after Universal closes for HHN. Then afterward you could go to City Walk. I doubt you are going to the parks, but that's honestly my best solution, as the place is crawling with scareactors all around the park, not just the haunted houses. Last time I was there, there wasn't much strobe lights around the park, but it could be different. Either way, I would imagine it's not that intense (strobe light wise) around the park, as I think they'd use that for mainly the haunted houses.
However, I do want to personally want to say that if this is strictly preference and not a medical issue, that it might be possible for you to survive HHN if you go without barely being scaved. You just won't be able to enjoy much. This advice isn't to just you, but to anyone who just might want tips on how to avoid being scared, or bothered at HHN.
When I walk around the park, I often move with a purpose and keep an eye on my surrounds, due to this I never get scared. The scare actors typically target people who are roaming slowly, stop, appear to be distracted, are talking with their group, or is busy doing something. I have only jumped once while walking around the park and it was due to one area being surrounded by heavy fog (which I only experienced once at the park), therefore I was literally unable to see when a scare actor walked up to me, and simply said, "Boo!" They usually do not waste their time on me. As almost all scare actors do not waste their time on me due me probably being fast moving and being aware of them. I always make sure to keep eye contact and look calm. Very rarely they might say something to me, but due to expecting the unexpected, it really doesn't phase me if they say anything. I've noticed they go for easy targets and want easy targets, and if you make it appear you're not one, you should be cool.
While you'll hear chainsaws and they do often move around the park, they are only in one area at a time (at least last time I was there). So, if you listen hard enough, honestly, you can hear them in the distance and you can just walk the opposite way.
Another thing is that there are Universal attractions that are open during HHN, if anything you can just run to attraction to attraction, and try to avoid the mess by moving quickly, knowing your surrounds, keep eye contact with any scare actors you might walk by, keep calm, and listen carefully on who might be near by.
If anything, try to stay inside restaurants and enjoy a nice meal. I heard (do not know if this is true) that a scare actor will sometimes (rarely) go inside a building. However, when I went to Flanagan's they were generous enough to stay away. Also, I think gift shops are open? If anything there's that too.
In conclusion, as others said, look up more recent videos of HHN to get an idea of what you're up against if anything, but if it is truly for medical reasons, it's best to just stay away.
