When I finally got a couple attempts to print using front fine art paper feed, I got bad head strikes and smudging in the first inch of the print (used thickness 6 and Wide platen gap).
I think this is a problem in any printer when using using sheets cut from roll paper if the printer thinks it's a sheet and not a roll. This is a problem in the 3880 using roll paper, and I assume also in the P800 if you set the P800 for a sheet of a certain length rather than a roll, which you must have since you were using the front feed.
In a printer with a roll paper option, the printer will advance the paper a couple of inches so that the paper is held flat by the exit rollers. If you start the print closer to the edge of the sheet than this then the paper curls up and you get the problem you experienced up to the point where the paper starts passing under the exit rollers. The solution is to have a leading margin of 2" to mimic what the driver does if roll paper is specified.
Note also that the same issue will probably occur at the end of the print unless you also have a 2" trailing margin. Again, if you were printing from a roll, then the trailing part of the paper is held flat by the entry rollers, and so you need enough of a trailing edge on a cut sheet to mimic this effect.
Can't help with the rest of your difficulties, my 3880 rear feed works fairly readily with sheets cut from rolls.