Your reply about the letters regarding the treatment of your query letter writer was perfect. No one is doing that person a favor by coddling him; how is he to learn even the fundamentals of writing if he’s allowed to skim on every basic? He is best positioned to gain by your direct answer, which was simply that: direct, and not unkind.
Regarding your response to the presumably teenaged person writing to you using text acronyms: I think you were perfectly correct in your response. First of all, if this person has aspirations of becoming a writer, he/she needs to learn right now how to appropriately write to editors and how to write PERIOD. Plus, this person needs to learn how to communicate appropriately in general. Whatever profession this person goes into, he/she will need to communicate with colleagues and it will look bad if this is how he/she communicates.
Second, you are also correct in your assessment of the younger generation. These young people seem to think that they are entitled to whatever they want, when they want it. Well, it doesn’t work that way, and better that they find out before going into the working world, where they are surely in for a very rude awakening. Sometimes a kick in the pants is the kinder action.
(I truly hope that this person was a teenager. If that was an adult, we are all in really big trouble.)