Well in the first place I wouldn't have stationed the GDL on Hesperus in the first place especially after what happened during their previous visit. If anything Katherine should have sent them to the Clan border where their experience fighting the Falcons would have been handy. They could have freed up a more reliable unit better suited for deployment to Hesperus II.
Another issue that I had was that both line units were stripped of their aerospace support. I mean seriously, the most important manufacturing facility in the Lyran Alliance and you strip the defenders of their aerospace support. While the warship was helpful, there is only so much of the space above Hesperus II that a single warship can patrol.
Third issue was that there was no clear chain of command. The 36th was semi-independent and was able to refuse orders that would have probably forced the Free Skye Forces to withdraw and reconsider their options.
Final issue, what were the 15th Lyran Guards even doing on Hesperus II in the first place? After the 14th Battle of Hesperus II, the 15th Lyran Guards were significantly understrength and were given little opportunity to rebuild as they were assigned to Operation Bulldog before being reassigned to Hesperus II. During that time another Loyalist, such as the 26th Lyran Guards, should have been permanently assigned to garrison Hesperus II along with the 36th Lyran Guards.
The 36th was assigned to defend the Maldon mining complex (of far lesser value than the production facilities). After they initially refused orders to redeploy to assist in the defense of the main manufacturing facilities (citing their standing orders to defend the mining complex), the 17th Skye Rangers were later deployed to pin them down. The arrival of the 14th Lyran Guards finally forced the 36th into the fight.