Hi, Homer jjj,
Try loading your video into a newly created project. The project resolution and frame rate will be set as that of the first video clip inserted into a newly created project. If the first clip is 1080p, then 1080p will be your permanent project resolution and you should get a 1080p export option. Deleting the first clip and inserting one of different resolution will not change the project resolution. Check your project resolution by clicking on the Settings button above the timeline to the right, and look at the top of the dialogue box that appears to see the project resolution.
Converting a clip to a higher resolution does not necessarily increase quality. Converting to more than one step higher, say from 720 to 4k can actually diminish quality. Also, quality seen in the timeline often does not reflect the quality you will get when you export the final product.
If still having difficulty, try opening iMovie while holding down the Option and Command keys and selecting to delete preferences in the box that appears. iMovie will open in a new library. Reopen your old library to get back to your projects. Now see if your quality and export issues are resolved.
-- Rich