iPhone headset won't fit

Working backward from the end, I'm afraid I posted here only because I'm desperate, having already chatted (after a half hour wait) with someone in support who no doubt searched her database, knew no more than I, and just basically told me that there's no service until Apple stores reopen in maybe a month if I'm lucky. (New York City has already been shut down over 11 weeks.)

No, I already searched online and found the advice to which you linked. I already scoured the port for lint and looked for damage to the jack, but then I thought I said that? And, as I thought I said, it also appears identical to another headset I have from a previous phone and to the adapter I have applied to fatter, bulkier old stereo headphones. And, again, damage is unlikely given that it fits snugly in a Kindle Fire port. As for checking the earbuds, that's of course for a different issue, volume. Anyway, I'd have said lint is out because the insertion seems limited by width of the jack as it tapers, not by depth of the port.

Well, if you've any advice I couldn't find myself, I'd honestly welcome it. Otherwise, I guess there's nothing to be done.

Nope. I don't actually have a case, although I know it might be a wise precaution against damage from dropping the phone, and anyway the entrance to the hole is totally unimpeded. Besides, that was in the link that I'd already read and the preceding reply had me reread.