Yes, it is similar to v/ve, but it (the vocalisation) is much rarer for syllabic prepositions. I can now only think of the example given by http://prirucka.ujc.cas.cz/?slovo=ode#nadpis2_2 (ode dveří).
It is something you learn by listening and reading as a child when you are a native speaker. Hard rules do not work well here. It is better to get used to the individual combinations and search the corpus if needed.
Earlier there was a sentence like "This is from all of your friends." Assuming we're talking about a gift, does the sentence in THIS exercise mean "I got one present that was from everyone jointly" or "Each person gave me something individually"... or both? Just wondering.
This should mean you got something from everyone. The same as "od každého".
If you got one joint present I would expect something like "ode všech společně" or "ode všech dohromady". Or you wouldn't say "něco", but you would be pointing to the actual joint present: "Toto jsem dostal ode všech." But even this sentence could mean you got more of these, each from everyone, the context should make it clear.
This one you got something from each of them. My guess is that 95% of people would understand it that way if not 99 and the rest would not be sure if it was one thing from them all or different things from each of them. The word 'something' or rather 'neco' kind of suggests the fact that was a different thing.