Is "uncured" bacon made with celery powder actually nitrite free?

No. Cultured celery powder is a concentrated nitrate source that a starter culture converts into nitrite before it ever reaches the meat, and that nitrite is chemically identical to E250. Independent testing of deli meats has found comparable nitrite and nitrate levels in products labelled "uncured" and in conventionally cured ones. USDA labelling rules require these products to say "uncured" and "no nitrates or nitrites added, except for those naturally occurring in celery powder", which reads far cleaner than it is. If nitrite is your trigger, celery-powder bacon is not a swap.

Do the nitrites in bacon cause headaches?

They can. Bacon is preserved with nitrite curing salts, and nitrites are a recognised trigger for headaches and flushing in sensitive people. That, plus the histamine that smoking adds, is why bacon bothers some people who tolerate other pork.

Are nitrite-free deli meats safe if I react to processed meat?

Nitrite-free versions remove one of the three triggers, but curing and storage can still leave them higher in histamine and tyramine than freshly cooked meat. If histamine or tyramine is your issue, fresh-cooked meat is the safer choice.

Why is sodium nitrite still allowed if it can form nitrosamines?

Because it is the practical defence against botulism. Cured meats are salted, low-oxygen and often held without refrigeration on a deli counter, which is close to ideal for Clostridium botulinum, and nitrite is the only widely approved additive that reliably suppresses it. It also prevents the rancid warmed-over flavour of stored cooked meat. Regulators responded to the nitrosamine problem by cutting permitted levels sharply and requiring ascorbate or erythorbate to be added with it, rather than by removing it.