The first thing to realise here is that that is not Old English. Read the quote carefully:
an *-ian verb-forming suffix in Germanic
That means the form is Proto-Germanic, rather than Old English. It’s perfectly customary – in fact it’s the standard convention – to use asterisks to indicate that a word or form is reconstructed, meaning that there is no ...
I've never seen this kind of vowel alternation analyzed as a "simulfix" like that.
I would say that it would be preferable to use either of the following analyses:
the different phonological forms of the words are simply stored separately in their whole forms: (/əˈɹɪθmətɪk/ and /æɹɪθˈmɛtɪk/). This is redundant (due to the common consonants between ...
• Are the two featured categorizations correct?
They do look correct, from my point of view. Unfortunately, you haven't mentioned whose exactly point of view you would like your categorizations to be judged from. Let's hope somebody else here can guess that.
• Why aren't class-changing affixes regarded as inflectional affixes?
Here I will speak about the ...