Ignoring all considerations of efficiency and possibility, would the FDA restrict a diabetic person from buying slaughterhouse scraps (pancrea), extracting insulin from them, and using that insulin to regulate their blood glucose levels? Are there grounds for such restriction? Would they consider restricting the scrap provider?
Via the Insulin Amendment Act, later repealed, and the Durham-Humphrey Amendment, the federal government gained the power to regulate insulin, and the Keefauver-Harris Amendment imposed regulatory requirements for proving safety and effectiveness. If you want to get in the business of manufacturing pork insulin (nobody in the US does: there are no approved US sources), you have to get FDA approval. 21 USC 331 prohibits introduction or delivery for introduction into interstate commerce of unapproved drugs.
You can argue that personal use does not constitute "introduction into interstate commerce". An analogous argument was tried in the case of Wickard v. Filburn, 317 U.S. 111, where appellee was penalized for growing and feeding wheat to his livestock, contrary to federal law. The court found that manufacturing for personal use potentially impacts interstate commerce, so the wheat regulations are not unconstitutional. Nevertheless, insulin-relevant law is framed in terms of introduction into interstate commerce, not just affecting interstate commerce.
The FDA's consumer guidance on importing beef and pork insulin is here. They state that you can apply for permission to import unapproved animal insulin, and they state that they will not generally allow unapproved importation, however
FDA staff may use enforcement discretion to allow the importation of unapproved drugs into the U.S. under certain conditions. One such condition is personal importation of unapproved drugs, like beef or pork insulin, that are medically necessary and not available in the U.S. If you are a person with diabetes who can’t be treated with human insulin or its analogs, FDA may permit you to import beef or pork insulin into the U.S. for your personal use.
Again, this is for person use only. The bottom line is that there is a basis for thinking that the FDA could restrict you, but not necessarily a sufficient basis to say that the restriction would be legal. The underpinning of these restrictions is the Commerce Clause, and the language of the restrictions is more narrow than the frequent "affecting interstate commerce" connection to that clause. While the courts generally accept that "keeping people safe" is a legitimate government interest, a narrow reading of the language does not support the theory that making your own insulin is illegal.
There would be no legal basis for the FDA to penalize the scrap seller.