It really depends on how intricate you scaling needs to be, I have not had a problem so far with re-scaling manually using the FX panel, but for more precise work i have always have After Effects out ready.
you shouldn’t have to do a video mixdown before exporting, what export settings are you using? dop you have the correct codecs for your Quicktime application?
how many layers does the photoshop file have? i have found that if it contains more than 5 the Avid struggles with import. How i got around this was merge a couple of the layers and rename the file.
when you import select same as source and the import should take no time at all as teh avid will not have to re-size, however the .mfx files can be huge.
i often just group using inpoints, just mark the clip and repeat on subsequent clips, then highlight all and group by inpoint. I’ve never had that not work
Sort your bin by resolution of the clips (apple E). Select the clips you want to redigitize, delete the media (just hit delete and UNCHECK master clips – you want to delete media ONLY). Then re-dig at the desired resoloution.