Are there any alternative admission gateways into J.D. programs for non-traditional law students?
I’m not sure what you mean. The only admission pathways of which I’m aware are through LSAC (a central clearinghouse for law school applications), which requires a college degree (or international equivalent) and an LSAT or, in some cases, GRE score. If by non-traditional student you mean someone who has been out of school for some time (like ten years or more), my understanding is that those students apply via the same mechanism but some leeway may be given for grades that are that old, especially in light of work experience.
No. You can only apply through LSAC.
Some leeway may be given, but they will ultimately consider your LSAT score the most, regardless of your other accomplishments.