None of this is something that hobby cryptographers can't do. In fact, there is no real difference between "professional" and hobby cryptographers. Many of the professional ones started as hobby cryptographers, and there are plenty of allegedly professional "cryptographers" who do not have sufficient experience in cryptanalysis. Moreover, anybody can make a fairly secure Feistel cipher, for example, it's just hard to create an efficient one. Note that proofs of security either don't exist or are based on unreasonable assumptions. Cryptography is still mostly a black art, not a science.

In a nutshell, you really shouldn't reinforce the "don't roll your own crypto" mantra. It just means you'll get less skilled cryptographers in the end. Bear in mind that nearly all debacles in cryptography were caused by professional cryptographers.

I thought this was pretty obvious but I guess this important context was not sufficiently disseminated given the prevalence of the latter position.