Really depends on the panel. Many are designed for top feed or bottom feed, others do have a side that must be the top. Most of the time is dictated by installed breakers - including the main. If any handle operates up-down there is likely a proper up side. If all handles operate left-right it is likely able to be mounted either way.
There is nothing wrong with splicing the feeder conductors as long as conductors and splicing method do not take up more than 75%(I think, may have to double check) of the cross section of the wiring space.
Often times those retaining devices only work in certain positions of the panel - quite often the ones adjacent to where a main breaker is intended to be installed.