Have you tried the approach "Loading Special Paper in the Single Sheet Guide" on P9 of that manual? I've used this rear feed with considerable success on similar printers to the R2400. The feed can be a little tricky on some printers, but generally it's manageable.
I don't profess to be an expert on the front feed on these printers, but my understanding is that the front feed is for very thick, usually rigid media. P11 of the manual says "You can load media 1.3 mm thick into the front path". The paper you're trying to use is nowhere near that. My hunch (and it's only a hunch) is that the platen gap is set very wide to accommodate that sort of thickness, e.g. printing on board, and that's what's leading to your smudged prints.
Have paper feeding mechanisms improved? Not sure. I think they're ok if you use the right feed for the right media. There was an issue on the P800, in that the rear feed was removed and you had to do what you're trying to do, i.e. feed via the front. People found this to be tricky but there was a workaround. I think I recall reading that the rear feed was coming back for in the P900. Don't know about the P600/700.