Sounds like your doing the best you can and by doing research on Betta and their proper care is awesome ! I bet these little ones were sick when you got them so hang in there and treat them as best you can and keep us updated on how they are doing. I have read most of everything you have written and dont really have any ideas to help you, I am a first time owner of my Betta for 6 months now and I am just learing as I go and this forum has helped me so much !
It could be that your water is different than what they were used too they ph could be different. Most will adjust but it could have caused them stress and stress leads to health problems. Of course there is not much you can do about that besides give them time to adjust keep their water warm not sure if you have tank heaters or not but their water temp should be from 76 at the lowest and up to the highest at 82.
Yes that is great food you are giving them so no problems there, I use it too and its a great source of protein which they need.
The IAL I dont know if that could be a problem or not all I can say is I use them too and ordered mine from overseas and I always just rinse them off and tear them in two peices and take off the stem since it is hard and kinda sharp. After that I just float them in the tank, I take them out after a week and put new ones in. Never had a problem. I have been lucky my little guy has been healthy for the 6 months I have had him. I wish you the best of luck !!!!