If you've really done enough core work correctly + mobility work try correcting your anterior tilt while looking into a mirror - by clenching your glutes & tensing your abd core while standing, you should achieve neutral pelvic tilt. And better yet, learn to brace while breathing, via abdominal breathing of course.
You might do all the core exercises in the world, but if you can't fire it when needed, you haven't learned the proper motor control yet. I'd say planks (side, front) +/or pallof presses are enough for a start, by just learning to know how to "brace" correctly your abs doing them, and then using that to brace your abs in your other exercises. The same bracing will spare your erectors a lot of work when deadlifting.
No, you shouldn't stretch the lumbar erectors. Use your "bracing". I really think that you're overanalyzing here.
1) look at the links
2) make effective use of the core work, by learning to use the abdominal core during the exercises and outside of it, continuing with the glute activation-mobility drills
3) start learning to do a RDL without loading, say a broom/empty bar, and progress from there while using your core to stabilize the spine, using the glutes for lockout
I've a friend whom we train together, and I've not allowed him to deadlift yet after 2 months, because of the same issues as you as regards to form. And I probably won't until he can get to do RDL's with the standard 45 lb's plates with good depth; so far he can RDL with good form with the 25's. Progressing slowly is not a bad thing if you're just lifting for overall health. You'll be happier in the long run.