Actually your own link asserts that you are wrong here. It says quite specifically that you use I for a subjective pronoun--in English grammar terms... that means in the "nominative" case. You ONLY use "me" (wherever you might think it is to be placed) when it is in the OBJECTIVE case... and in none of these examples is "me" objective... not the object of a preposition, and not the object of a verb. Actually the final "I" is the SUBJECT of an elliptical clause: "He is reading slower than I (AM). The verb of the subject "I" is understood--but putting a "me" there is most definitely incorrect English grammar! Any time you make a comparison such as: "I am faster than (he/him)"... the CORRECT choice is HE because the word is the subject of an elliptical: "I am faster than he is" The same holds true for I/me. Just an FYI.
No. Comparisons with "als" and "wie" normally use the same case as the noun/pronoun they refer to. Here, "als ich" (literally: than I) refers to the pronoun "er" (he), which is the subject of the sentence and therefore in the nominative case. That's why "ich" is in the nominative case as well. ("Mich" would be the accusative case).
You wouldn't say "Er liest langsamer als mich", not even in colloquial speech. However, the accusative is used in a comparison if the thing you compare it to is also in the accusative case. E.g. "Er mag dich lieber als mich" (He likes you more than me").
See my explanation above.