I tested the combination of HEC and sodium polyacrylate. It was a simple test to see what happens when I mix the two (disastrous or okay).
Temperature 4.6 ℃, humidity 82%, early morning
The solution was made with the following recipe. (The amount is somewhat arbitrary)
・ 0.7g Sodium Polyacrylate
・ 5.3g HEC (Natrosol 250HHR)
・ 1,000ml hot water (Boiled)
・ 3,334ml tap water
・ 8.0g baking soda
・ 1.3g citric acid
・ 240g Cucute (Japanese detergent)
I added mixed powder of HEC and sodium polyacrylate to hot water 1,000ml little by little while stirring with a magnetic stirrer (and after that, I added baking soda). After stirring with a magnetic stirrer for about an hour, I saw some lumps (It was clearly due to sodium polyacrylate). I added tap water 3,334ml, citric acid and detergent to it, and let it sit overnight. Next morning, lumps had disappeared.
I went to river beach and tested it. The solution was friendly and it was easy to make a spherical bubble of 1.5-2.0 meters. It was difficult to make spherical bubbles larger than that, which may be due to the small size of the tri-string wands I like (it was easy to make floating tubes 5.0-6.0 meters).
As far as I can see, sodium polyacrylate has no problem mixing with HEC.