Probably the best way to do this is by making the merging more...symbolic. It can still be pretty nasty, with a correspondingly high chance of infection, death, chance to show off manly scars, but it might be somewhat survivable, whereas yours is...less so, and will quickly die out with any tribe that adopted it.
So how it would work is, you would have each helmet crafted to the specifications of the person coming of age - possibly by them, or with their involvement, maybe with a ritual purification, a blessing, certain rituals, etc. This is going to be their helmet, forever - it should show status, it may have ways to add later acquired symbols of status (prowess in battle or the like), it may be heavily decorated, engraved, and symbolic.
For the actual ritual, you may want your warriors to have their heads shaved, or maybe (increasing survivability again) have them shaved a set amount of time before the ritual, so there's an even stubble of hair at the time, since some heat would be spent burning the hair rather than flesh. There will likely be ritual cleansing, maybe fasting or in the other direction, feasting (with lots of alcohol), or other religious/social rituals surrounding the day. As their initiation, each warrior will be branded on their heads by a red-hot helmet (possibly in privacy). They may or may not have rough medical treatment before putting on their helmet, and going out to display their new status symbol.
Practically, it would make sense to have a specific helmet just for branding, it would be easier to make to spec, to replace when damaged, and less problematic if the branding helmet cracked under thermal stress (likely, over time, to happen to someone). Symbolically, this would be about marking them as equals, branding them as warriors of the tribe first and foremost, and only later given their individual status as the helmets they display. On the other hand, you might want to play up the "merging" of warrior and helmet, if so then it would be their own helmet they were branded with, to closely link themselves, flesh and bone, to the metal of the helmet and symbolically "imprint" them to their warrior (though in this case the helmet must be cooled before being worn for display).
Anyway, they would display themselves wearing their new helmets, and some time afterwards (how long depends on all sorts of things) they would go home and be given proper medical treatment, including bandages. Their scalps would be given a chance to heal before they must wear the helmets into battle - though perhaps they may be required to wear them in public for a time/forever to further the symbolic merging - and the time to heal and access for treatment would reduce the likelihood of infections and other nasty consequences.
As far as pre-initiates or outsiders are concerned, the helmets were (permanently) attached to the scalp via burning. If the helmets must be worn in public for some time, or ever, this would only reinforce the notion. And there are genuine burns under their helmets, which would only reinforce the idea if someone were to see a person without their helmet for whatever reason in private. These symbolic helmets may or may not be the same as the helmets they wear into battle. Status symbols are usually for show rather than use (since breaking them is bad), and you would need some ritual way of re-attuning a new helmet each time the old one is damaged or breaks past usefulness in battle. If the helmets are different, the status symbol can be kept, likely for life, and it can also be a great deal more ornate and symbolic - this would be the case if important thing was wearing a helmet, rather than the specific one. On the other hand, if you're playing up the symbolic merging, it could be that no other helmet will do because it is "mystically bonded" to its warrior, depends on how religious vs social the ritual is.
In any case, what the ritual would actually be doing is just branding the person - which will show off stoicism, strength and resistance to pain - which is a fine and survivable manhood ritual. Either they will be branded as warriors of the tribe, and later take up their individual status, or else they will mystically bond to their helmets through this ritual. It will also probably reduce or eliminate head hair (which may be desirable, as it "shows" that the warrior is always ready to wear their helmet and do battle, or else emphasizes that the head is not "complete" without its helmet). The conception that this merges the helmet to the head can be accomplished by having the new status symbol be mandatorily worn in public for some time, coupled with the scars if or when someone sees a person without their helmet. There would still be some losses to infection or other complications of the procedure, which would play into the seriousness of such a ritual and also the myth (since merging the helmet to one's head would also be expected to produce fatalities).