I have been marbling silk scarves for a couple years now and here is what I have learned:
1. Haboti or China silk is the best for allowing the color to show thru. It's not as clear as the marbled side but looks beautiful in its own way.
2. Use pre dyed scarves like one can buy from Exotic Silks which seems to be the wholesale arm of Thai Silks. This prevents the white boarder problem you mention.
3. Wash the scarves with Synthropol before you add Alum by putting 4 or 5 of them in a zippered porous bag. I usually wash 5 or 6 bags at a time (so 20 to 30 scarves). This prevents the scarves from tangling around each other and around the center post, if you have that kind of washer, and prevents the creasing you mention. Also stop the washer just before the final spin. Just hang them and let them dry. This helps prevent any creasing also.
4. You need to rinse the scarves lightly a couple times right after marbling and then leave them alone for at least a week before you do any vigorous rinsing. My guess is that the stiffness you are getting is from the carrageenan that has not rinsed out. However to much paint can cause some stiffness.
5. Acrylic paint is a plastic and once it dries for a week or two it's not going anywhere unless you did not wash with Synthropol so there was still sizing on the scarf when you started. The paint never really bound to the fabric and flakes off when you wash.
6. I recently sent some test scarves to the dry cleaners and they came back fine and beautifully pressed. However I still recommend that people hand wash them with very light soap.
Hope this helps