1:Marx did not invent the idea of socialism - that is an important fact.
2:Productive relations are social relations, you cannot change the social relations of production without changing productive relations.
3:If, as in 'Wage Labour and Capital', Marx is correct in assigning our labour as being our life activity, then we cannot claim socialism to be about freedom if it involves forced labour.
4:I do agree though, that if a vital task is so universally unpopular as to not get done, then indeeed aquitable rotation might have to be considered, but again, I consider that a fall back plan, rather than a first principle.
5:Dictatorship of the Proletariate, simply means the dreaded old tyranny of the majority, i.e. fully functioning democracy - we outnumber them vastly, we have no need to inhibit their voting or speaking rights.
6:Capitalist social relations are highly developed, with virtually no peasants or petit bourgois to contend with, so the DOP could last a very short time indeed, enough time to expropriate the means of production, basically.