Concerning your remarks :-)
Concerning the equation:
Given in the slides: ci = pi + c(i-1) mod' 25
You want to correct it to: ci = ci + c(i-1) mod' 25
Please have a look here, you don't have ci but you want to comput it. So this is not possible with your equation. In each step after the first two, Josse encrypted each plaintext letter using the previous ciphertext letter: ci = pi + c(i-1) mod' 25
2) Concerning the alphabet:
Yes, you are right. The original Josse cipher uses a 25-letter alphabet (excluding the W). The CT2 component is a bit more flexible. The default the component uses is the 25-letter alphabet, but the component allows to provide your own alphabet. And the template provides an alphabet with 26 letters (including W). We added the W in the template's alphabet to allow the W in "world" being encrypted. In the challenges, we used a 25-letter alphabet as described and as Josse used. If you want to use the CT2 template and it should work as the original cipher, you can either delete the W from the alphabet or delete the TextInput component providing the alphabet.