If you properly set up your project for AVCHD or even MPEG footage, you don't need to mess around with any of these things, JMB. They're handled automatically by the project preset. (I can't imagine any workflow that would involve manually changing the field dominance one clip at a time!)
When you started your Premiere Elements project, which project settings did you select?
I have project setup for AVCHD (1920x1080) and plan to output bluray. So this assumes high-field 1st. Thus I did not need to alter any imported videos from my .mod or .mts files since they were high field first. However, all my .mpg files (lower field 1st) needed individual modification to set the reverse field dominance. No automatic stuff.
I know my .mpg files work correct as is in other players (WM & VLC) as well as play nicely in PE organizer and PE when doing DV preset.
You would hope that when you choose an AVCHD preset (hi-field 1st) and it should know the .mpg files are low-field 1st, then it would automatically reverse field order them, but nope.
You're mixing AVCHD and standard def video in an AVCHD project? That's not a wise workflow.
I also don't know any reason you would have to alter your field dominance if you are using AVCHD in a project set up for AVC HD.
Is this standard AVCHD from a camcorder? What model of camcorder is this video coming from and which settings was it shot with?
Something just doesn't make sense here.
I indicated above that I have no trouble with AVCHD inputs (.mts).
What's wrong with mixing .mpg stddef videos with AVCHD? I created a project that shows multi-year footage. Some of it is HD and some of it is old stddef 8mm/DV (.mpg). Actually, mixing stddef on HD output is great, because you have so much resolution to work with. I can actually watch 4 stddef videos on the same screen w/o losing track. Such intense display of material is quite entertaining and you can't take your eyes off trying to catch everything.
On a side note, I created this 2 hr blu-ray project (AVCHD output of ~20GB) using hundreds of images, stddef & hddef videos, and up to 12 stacked video channels. The encoding process only took 2 hrs and used a max of 20GB of ram (32GB installed). Very happy with 64-b PE10!