-A socialist economy is 'centrally planned' in the sense that there is one plan that regulates production on the societal scale and determines its total product. Increasing popular 'participation' doesn't make the plan, which as said is the regulator of production under socialism, any less 'central' or 'pure'. What 'pure' central planning does
rule out, however, is allowing local or enterprise-level interests determine the social product. Concessions for local interest can be made, for example to balance out remnants of national oppression, but these are made within the framework of the plan. Therefore popular participation and national emancipation are compatible with centrally planned socialist economy, while enterprise-level "self-management" fits only the small-scale production as a transitional measure during the construction of socialism.
Planning is a key feature to any major economy, including under capitalism, but if the decisions of the economy are taken away from those who produce society's wealth, there runs a risk of the same problem under capitalism: alienation from one's own product.
Yes, but Marxists don't seek a solution in some revival of petty-bourgeois or artisan production in a cooperative form - except where it presents a stepping stone from such small private production that hasn't yet reached the level of productive forces required for property of the whole people. Also, planning is a key feature in all production beginning from the smallest unit in any economic system.
There can be all kinds of economic plans under capitalism, but they can't be unified to the extend that they'd replace profit as the regulator of production on the societal scale. The planning of economy under socialism is qualitatively different from even the most nationalised capitalist economy, and planning of the level realised in the Stalin-era USSR is simply incompatible with any form of capitalist political system. As Stalin himself said:"In your system, gentlemen fascists, to whom do the means of production belong? To individual capitalists and to groups of capitalists and, therefore, you cannot have genuine planning, except for bits, as the economy is divided among groups of owners."
a system where the economy is fully in the hands of "the party"
I think such a system and an omnipotent party would be supernatural and theoretically impossible for Marxist class-analysis. Parts of the 'superstructure' can't become their own 'base'. State is a political system maintaining
class dictatorship i.e. ultimately a certain mode of production, neither
of which it can become
. Same thing with parties. Even the most powerful party can't be more
than a vanguard of certain class interests. In order to rule, especially if it is to rule alone, it has to lean on certain big enough class interests or bloc of class interests - again some material factors which can be utilised but not spawned from the superstructure itself.