Hi Bob:
2) The RIP will interpolate your image to the output DPI profile for output you select regardless of what size image you send it.
1) Hence there is some 'logic' that one should use a direct multiple of that output number before sending it to the RIP, thus making the RIP's job easier on the interpolation. With the Epson x600 and x800 I can concur that doing so made a small, yet visible difference in the final image, hence I adopted the routine of sizing my originals to 360PPI, 240PPI or 180PPI for larger and larger prints before sending them to the printer. (And it extended that Canon and HP printer users should use multiples of 300PPI for the similar reason.) I carry on with and continue to recommend this tradition today, however my gut tells me the RIP interpolation is good enough it's probably irrelevant for most prints whether you send them a 360, 300, 200 or 180 PPI image.
2a) I have tested 1440 and 2880 output on my 7900 and honestly can only see a hint of a suggestion of increased sharpness from 2880 in a 24x32 print if I view the deepest shadow detail section under a 16x loupe and squint, and do all this before my second cup of coffee. And then it's still only a hint of a suggestion of improvement
))
So in conclusion, I routinely work my images up at 360PPI and leave them there as my working copy. I still prefer to flatten and resize my print for final output size at 360 before sending it to the printer, as it makes it easier (for me) to interpret my optimal output sharpening regime for the intended output size. I then print at 1440 for speed and convenience. While we're on the subject, I've also compared bi to uni-directional outputs, and find virtually no significant difference in quality there either, so I routinely use bi-directional for added speed. This is on the 7900, and this was NOT the case with my 7800. Oh, also ink consumption was rumored to be significantly higher for 2880 than 1440 with Epson, but if that was the case, it is no longer the case. It's a small enough percentage more now to be insignificant, at least with the 7900. Regardless, I think the reality is these differences are subtle and you will never notice them with the newer printers unless you compare the final outputs side-by-side and under a loupe.
Cheers,