I think that to explain etymology of word 'comrade' by changing the first vowel 'o' to 'a' without notice of so common prefix 'com' is less than ridiculous.

So, let's with blessing from William of Ockham to decompose this word the simplest way: "com-rad-e", where 'com' is very common prefix and 'rad' is the root.

Wikipedia lists following etymology of 'rada':

1. Old High German rāt (from Proto-Germanic *rēdaz) passed (possibly through Polish) into Czech, Slovak, Ukrainian, Belarusian and Russian languages.

2. Råd in Norwegian/Danish/Swedish and Rat in German, Raati in Finnish and Raad in Estonia/Dutch means "council" or "assembly" but also "advice", as it does in East Slavic (except Russian) and West Slavic, but not in South Slavic languages.

3. In Swedish the verb råda (to council) is based on the substantive råd. This is similar to Danish; "råd" (noun) and "råde" (verb).

Ergo, comrades are those who counsel each other. On equal terms may, I add.