As a future licensed medical professional (hopefully here in a few months) with some exclusive training on concussions, I'll give you my take.
Its really like anything other injury as far as long-lasting effects and your ability to be symptom free. It depends all the factors both big and small in your treatment and recovery process, along with your own personal individual variable. The time in-between concussions, (months, days, years) the severity of the trauma (clearly your first and second were pretty severe), how many sub-concussive impacts occur in-between concussions, which will re-aggrivate symptoms, and how you manage yourself when recovering from a concussion. (Clearly you are smart enough to care about your brain, I give you A+). And of course, long term outcomes are where you the the CTE stuff and whatnot.
I am going to a sports med conference that has a whole day dedicated to concussions, so If i come back with something useful for you, I'll let you know.
For you I'd say honestly take a break, with your history of them it sounds like you'll have some symptoms. Take care of yourself, avoid computer screens, things that strain the eyes, high mental stressors, you know the drill you've had 2 of them. Try to minimize the symptoms and ease back into sending shit.
The next step for helping your symptoms or residual side effects, and answering long term concerns would be to see a Neurologist. They can get some accurate brain scans, only really useful if you have huge mental deficits or right after your concussed tho. And can prescribe different treatments, such as botox, meds, even PT. Accupuncture I've heard works great for some, go to a massage therapist and get a full occipital release (Neck musculature) which could be very tight or in spasm due to the impact, and can cause some of the longer lasting tension headache and migraine like symptoms. If anything use a hot pack on there, because that shit feels great.
As far as the helmet goes, it's really there to prevent fractures. It's a brain bucket. The coup and countercoup mechanism can't be avoided unless the helmet takes some of the forces and distributes it across the helmet away from the brain evenly without the brain moving (if that makes sense). The MIPS system is certainly better with the sliding mechanism taking away some of the forces. But it's to a point. I have no idea what kind of clinical research is done wtih it. So if they say they prevent concussions I'd be wary. I'll end my rant with a video link to what some NFL Helmet Prototypes look like, around 4 minutes mark is the model.