ङ & ञ have basically been discarded in standard Hindi spelling and replaced by the anusvara. So there should be a ॉं above the shirorekha, but ॉ is a non-traditional, relatively new introduction to the script for English loanwords and my impression is that due to ॉं occupying excessive space and potentially appearing as chandrabindu when typed, the spelling लॉन्च is used instead. Since “launch” is not part of the traditional lexicon it is not bound by traditional spelling rules.
More to what I believe is the point of your question, I think लॉन्च would be wrong in any context, since न does not represent the proper allophonic sound [ɲ] to precede a palatal consonant.
Just because a conjunct exists, it doesn't mean that it is phonetically possible, and least of all homorganic, even in Sanskrit-originated scripts and languages.
Even the Cambridge Dictionary uses लॉन्च, for the definitions where this loanword is used. So I think that the non-homorganic spelling is popular even though it may not be correct by Sanskrit grammar rules.