Example of how q primes help to reduce the search space.
First pic attached is for c25185549107 and shows summary output for the iterative search process at n/4 performance. The solution n=353827 is found in 885458 iterations.
Second pic represents gcd(a[t],c) solutions for different q*c products.
Search methodology here is simply iterating t in each (e',1). The t column is the grid index and number of iterations.
Tests run using various combinations of primes. (Those ending in binary 01, 11, and either).
Interesting that the best result for all these tests is 224 iterations and uses any prime.