First off, Elfboy, for one of your complaints, there is a half solution:
For Melee weapons, at least Ifrit and Alastor, you can swap between them while you're on the ground by pressing the L/R3 button (Forget which offhand); again, gotta be grounded, and you can't be in DT. It would suck that it only works for Ifrit and Alastor, but really, those are the only two melee weapons you should be using (Force Edge is worse than Alastor in every way, Sparda's a total novelty weapon more than anything else), so you shouldn't care. Guns, sadly, no shortcut exists, which is annoying cause I do find myself swapping between the 3 Main Guns a lot.
Anyway...
Actually, DMC1, when it was still Resident Evil 4, was going to be about a Detective (named Dante) going into a gothic castle and well, ZOMBIE OUTBREAK!!! there and all that. Just while making the game, they realized how little the game was resembling a Resident Evil, so said "Fuck it, lets keep what we have, but make a totally new game out of it!"
Xenogears allegedly was the same deal; initially meant to be CT's sequel, somewhere along the way they realized they were straying too far away from their intent, but they liked what they had already, so rather than scrap it, just alter it into something new entirely.
Really, DMC1 initially being an RE really shows. It has the same Camera Angles as REs, a shit load of emblem puzzles (though, DMC3 didn't fix this but I don't think it was as bad as DMC1), Marrionettes being the basic enemy sort of scream "These were originally zombies" based on how they move and such, enemy specific fatalities, the "You are Dead" game over screen...really, the game is just so obvious where it got its routes from.
DMC2, they actually worked away all the RE-ness from the game, though many will credit DMC3 with this, cause DMC2 was just so different than DMC1 that it might as well not count (where as DMC3 has a lot of the same basics as DMC1, just with a lot of improvements, modernization, etc.)
Interestingly, DMC2 was the opposite compared to DMC1 in regards to the weapon swap. You could swap Guns on the fly, but not Melee Weapons. Then again, DMC2, the 3 weapons were identical for melee in terms of how they handled, just different Range and Power, so you typically just stayed with whatever you found most favorable, where as Guns were about as varied as DMC1.
Then DMC3 just lets you swap both! ...but limits you to one style at a time, sadly. DMC4 then said "Take this Style Swap button and have fun!"