Hi Quixote,
If you have worked out 3dn and 4dn, and have a good idea of 5dn, you must know what prime number is represented by G. If you don't, you can deduce that G must be a single-digit prime, as 5dn has three digits and so do 3dn and 4dn, which are basically 5dn multiplied by G. If G had two digits, then 3dn and 4dn would have four digits.
You already know two of the four single-digit primes from 7ac (C and H). You can eliminate one of the other values easily, because GKL+/-C will either contain a zero, which is not possible as the grid contains only the digits 1 to 9, or either K or L will have to equal G.
This means that there is only one possible prime value for G, and as 6ac is ACDG, you can work out what the final digit of 6ac is - you should find that it is the same as the second digit of 7ac. Given the placement of 7ac in the grid, the answer to 6ac can go in only one position, and this limits the possibilities for the pentomino containing the second digit of 7ac. As you correctly note, the second digit of the two-digit entry at the top of the grid cannot be the same as the second digit of 7ac, or the entry would not be prime.
I hope that this gets you to the next stage. I found that there was only one possibility for the pentomino containing the second digit of 7ac and this meant that there was only one possibility for the pentomino containing the fourth digit of 7ac. If you can work out the shape of 6ac, in the only position it can occupy, you have just enough information, I think, to divide the whole grid into nine distinct pentominoes, and the other numbers fall into place quite quickly.