Need help aligning all my parts to a plane- advice appreciated

its much easier to figure the issues with the actual model rather than image and words. you can upload via a private message if you wish, Otherwise its kind of a guessing game where the problems lie,
At a guess, if the pieces are components then its an axis issue, If not then (in my opinion ) they should be components with axis aligned appropriately,
Objects can be moved absolutely [x,y,z] or relative <x,y,z> see IFC Model imports - #3 by DanRathbun

If you had the professional version, and for those who do, then the dynamic attributes is a quick way to move and rotate the objects to a plane.

So maybe set up a XY plane at 0,0,0 and make it a group and lock it will suffice as a reference plane, move an object [0,0,0] then rotate to suit…too much work

There is an extension called Align created by D.Bur that will make this operation simple, but note its based on the proper alignment of a component. this can be down loaded via window menu…extension warehouse

Maybe I have been able to identify the issues you are having, if not then please send me that private message