Question about singing low notes/tones?

you are totally confused, and I think the diaphragm thingy is irrelevant here. Low notes below your lowest note, that's sung in the vocal fry register. With enough practice, you can learn how to use vocal fry, maintaining pitches and maintaining a recognizable sound wave.

First thing, forget about the mouthyness. Vocal fry is throaty, and you can achieve the proper sound without using the mouth. the mouth will just ruin things for vocal fry.

It's hard to recommend any tricks for vocal fry, but two points that "might" help, is that: 1. you can imagine your voice tilting downwards (towards your chest/lungs) while going into these low notes, because physiologically, the muscles do actually move downards slightly. 2. Playing with a "Mi" (mee in english) vowel/sound, will help form the vocal fry sound more easily.

But those 2 things are only.... sensations. It does take time to develop vocal fry, and it should NEVER hurt. If you are trying to force vocal fry and get ANY irritation,even for one picosecond, that's extremely dangerous for your vocal cords.

and on a last note, if you plan on being a pro/high singer AND singing in low vocal fry, it's basically necessary that you have a naturally low voice. While high voices can find/use vocal fry, it's impossible for them to perform in it due to the muscular requirements of good pop sing, or at least, they can only use it before they get proper training.

The lowest note I've sung in vocal fry, with a recognizable sound wave, was a D1.

This shows very clearly that self-help DOES NOT WORK.

In fact, it takes obviously some talent, some luck, LOTS of patience, diligence, courage, hard work, dedication and LOTS of lessons with a GOOD vocal teacher. Besides, your voice will keep maturing until you are in your mid-30s so you need a lot of TIME as well. Therefore:

Sorry, but THE ONLY SAFE way to learn the correct singing techniques & to improve properly IS to take OFFLINE face-to-face singing lessons with a fully trained vocal teacher! The teacher HAS TO BE in the same room with you, so that he/she can give you proper feedback. However, even the best teachers in the world cannot make wonders, so please be realistic with this. Singing lessons are NOT going to help if one is tone-deaf! Please do NOT rely on any dodgy web tutorials because that way you can misunderstand things VERY EASILY and develop bad habits, hoarseness, vocal nodules and other nasties IN NO TIME, and even though you would sound good! It is always much wiser to invest a little bit of your money/time to face-to-face lessons rather than wasting the same amount of money (or even more!) to frequent ear-nose-throat specialist visits due to aforementioned problems, so please reconsider this. If you can't afford vocal lessons, then joining a choir is the only SAFE alternative option. And believe me, but even MANY of those who have music as their hobby DO take lessons as well! Always remember to warm up your voice properly, but please know your limits and don't overdo your voice! Remember the diaphragmatic support, do not strain your throat too much! Also, remember good body posture! Avoid fizzy drinks (burp danger), dairy products (mucus risk), caffeinated products (coffee & tea included, they dry up your throat) and spicy food (irritation risk)! You can consume these things, but NEVER before singing! Do NOT shout, yell, scream nor otherwise abuse your voice AT ANY TIME! Also, please respect your vocal range; if your teacher says you are, say, more of an alto (baritone if you are male), then you ARE more of an alto (baritone). DO NOT try to imitate anyone famous, that will usually give you just bad habits and even damage your throat. You are YOU and your voice is unique, so please learn to cherish that. The world does NOT need copycats. Do NOT sing, whisper, shout, yell nor scream if having a sore throat/cold/flu, Also, do speak as little as you can if you have flu/cold/sore throat! Remember to drink at least 2 litres of room-temperature still water every day, not just during singing days! Smoking is a big no-no, as is inhaling secondhand smoke. Also, avoid staying in dusty and/or moldy environment.

There you go, leave it or take it, but self-help is as DANGEROUS as trying to perform a minor surgery on oneself after watching how it is done in an episode of Chicago Hope or Holby City.