'All (that) I read were a few magazines' or 'All (that) I read was a few magazines'?

The first one sounds strange to me, although the subject 'all' can be plural, can't it? 'All were happy with the new plan.'

Is it because 'all' really means 'the only thing' in those sentences?

How about...

'All you need to do is whistle.' or 'All you need to do is to whistle.'?

The verb form after the 'is' seems to be dictated by the verb in the clause describing 'all' - 'All you need is to be able to whistle.' 'All I enjoy is whistling' (need + infinitive, enjoy + gerund) - but what happens when we use 'do'?

'All that I read was a few magazines'. "That is all that I read." In both of these examples 'all' is being used as a noun of singular completeness, and the correct verb is 'was', in each case.

"All are welcome", is treating each person as an individual, whilst still collecting them together without losing the sense of that individuality, male, female, Muslim, Hindu, young, old, etc.

'All were happy with the new plan', again gathers the individuals together without their becoming one identity.
'The mob is becoming uncontrollable' reduces the individuals to one homogeneous mass, and gives a different slant from 'The mob are becoming uncontrollable', which still retains a sense of the individuals' separate identities. In this last case either will do, but as we can see the degree of menace is higher when the singular form is used as, implicit in the statement is the realisation that we can no longer appeal to the individual conscience to defuse the situation.
As you correctly say, " All you need to do is whistle", correctly translates as "the only thing".

'A noun of singular completeness' - I like that . Thanks, Ritzmar; that's a clear explanation that seems to make sense.

You are most welcome...