This is a complete shot in the dark, but it may be worth a try. On the ASSUMPTION that fewer sequences mean less burden on memory requirement (hence avoiding the green/black screen effect) – and I don’t know that this is the case, I might hastely add – why not create 7 appropriately labeled Bins, move your stills to their corresponding Bins, then delete the subtimelines (sequences).
As far as I can tell, edits and effects that you may have applied to the clips in the subtimeline will remain in the master timeline after you delete the subtimeline (assuming you copied them from the subtimeline, as you say you do). The separate Bins also serves to sort your stills, and you can view their content in icon view, or in the Source window where you can edit before bringing to master timeline, as was the function of the subtimelines.
Try it and see — BUT ON A COPY OF YOUR PROJECT, naturally. And let us know if this works.
Cheers