Another partial answer here... for now!
Going based on @Techidiot's answer@Techidiot's answer, I was able to discern that you need to:
Fill the second message with... something (maybe based on 2 and 8), and then read every third character plus two.
Without doing any "improvement" of the second message, that gives you this:
AR_#T_RT#W_TH_PRIMES_SQU__ES#AND#TRIANGLES#PLU#ONE#FOR#AN#ICON
Which I was able to partially translate to:
AR_ START WITH PRIMES SQUARES AND TRIANGLES PLUS ONE FOR AN ICON
(I'm unable to discern the first word – there's a few possibilities.)
Given all that...
... it's likely that the hint refers to indexing using prime numbers, square numbers, and triangular numbers, possibly with one or more of those incremented by one (2, 5, 10 rather than 1, 4, 9). However, I haven't been able to make any kind of recognizable icon in this fashion.
Taking a decent break from this, hopefully someone can come up with something in the meantime!
Based on other answers people gave, I was able to fill in some of the blanks from the second message. However, I noticed that some of them conflict – I filled in the sentence I solved with red, and the others with blue.
I don't know if it helps at all, especially with those conflicts, but I figured it was worth sharing.
