Perhaps it'll help if you understand where your mother is coming from; that'll help you counter what she's doing. Your mother is trying to make you do what she thinks is best for you. She wants you to be like her, but if you're not like her then she can't possibly understand what you're going through so no wonder she's making mistakes.
If you only tried to come out to her that one time a year ago and haven't actively reinforced it since then, she probably thinks it was either crazy talk, or it's just yet another phase you're going through. She probably believes you weren't serious about it, or you've already gotten over it because you're not banging on about it all the time.
She knows (and is correct!) that it's very difficult to be trans. She knows it may put you at a distinct disadvantage as you go through life. So she's trying to discourage you from making what she considers to be a huge mistake, and she's trying to make you do things that'll keep you on what she considers to be the 'right' path. That's why she reacted so badly, and that's why she's trying to force you to conform to female stereotypes. Most women would be horrified at the thought of having a hairy upper lip, so she thinks she's doing the right thing by making you get rid of those hairs. She's acting out of love & concern for you, but she has no clue what it means to be trans so she's doing all the wrong things. When you do come out to her, you're going to have to be her native informant.
Now that you have some idea of what she's thinking, here's what you need to do. Get ready for some (ironically) straight talking:
Harry, you're 17. That's just a couple of months away from 18, at which point you'll be a fully-fledged adult with all the rights and responsibilities that come with it. You're not a child any more, so it's time to get yourself out of that mindset and start enforcing your right to become an adult. Nobody ever gives you permission to be an adult; this is something you have to do for yourself. It's time to stop thinking that you have to obey your parents and that you need their permission to do everything... and it's time to start just doing things for yourself, independent of your parents. So:
Quote from: Harry_1 on March 26, 2017, 05:53:36 AM
I'm FtM and live at home.
If this is proving to be a problem, get in touch with various housing associations, your local council, the Albert Kennedy Trust, Stonewall Housing, etc. etc. and find your own accommodation. Move out as soon as you're able.
Quote from: Harry_1 on March 26, 2017, 05:53:36 AM
I tried to come out to my mum a year ago and it was disasterous so I am trying to find alternatives to coming out, or postponing it.
You do not need your mother's - or anyone else's - permission to be trans. Coming out is simply stating facts. It's not open to negotiation; it's not a way of begging for medical treatment; it's not something you need to wait to do to get moving with your life. You don't need anyone else's consent to live your own life your way.
Quote from: Harry_1 on March 26, 2017, 05:53:36 AM
The other week I shaved my (already quite prominent) mustache hairs and they have now grown back darker and thicker, as I expected.
You may be disappointed to learn that shaving doesn't actually make your hair thicker; it just makes it seem thicker for a short while because body hair is thicker at the root than at the tip, so a shaved hair will just have a slightly thicker circumference at the tip until it falls out... and is then replaced with your normal hair which is thinner at the tip again. It doesn't give you any more hairs, and it doesn't encourage them to grow thicker. Still, shaving is a typically masculine activity so simply doing it can help with dysphoria. And shaving off the bum fluff on your cheeks & chin will also help you look & feel more masculine.
Quote from: Harry_1 on March 26, 2017, 05:53:36 AM
This morning my mum asked if I've been shaving, and I lied and said no.
It's your face. You can do with it as you please. You don't have to lie about shaving, but you don't have to tell her anything either. It's not actually her business.
Quote from: Harry_1 on March 26, 2017, 05:53:36 AM
She said we'll have to get my lip waxed and to get out of this I said I'll pull out the hairs myself.
Tell her 'no thank you'. Tell her you like the hairs there and you are not interested in having them removed. Thank her for the kind offer of a waxing but tell her you will not be taking her up on it or on any other offer because the hairs are staying exactly where they are. She can try to apply pressure, but you can resist it and she can't physically force you to do it. Stand up to her. I'm in my 40s; do you think I tolerate anyone telling me what I can & can't grow on my face? Would your mother tolerate it? Heck no! And that, my friend, is what being an adult is all about. You're almost there yourself; why not get started now?
Quote from: Harry_1 on March 26, 2017, 05:53:36 AM
She's refusing to accept that this issue is anything to do with my gender.
Doesn't matter what she thinks. You don't need her permission or her validation. It's your life; it's your gender; it's your face. Do what you will.
Quote from: Harry_1 on March 26, 2017, 05:53:36 AM
If I don't remove the hairs, I'm worried she'll get more concerned about this, and perhaps stop letting me keep my hair short, etc.
She doesn't have that authority
unless you give it to her. She wants you to believe you have to do what she says, but you are not a child any more. You can have your hair however you want it to be, and you can do what you want with your face. Just tell her that you've made your decision & you're not prepared to enter into any discussions about it.
Quote from: Harry_1 on March 26, 2017, 05:53:36 AM
I won't be able to defend not removing the hairs without bringing my gender into it, and I absolutely will not come out before I leave home because it will ruin my life.
You basically have three choices:
- Stay in the closet until you're ready to move out. This is not recommended because in that time, your dysphoria is likely to continue to grow and it could get to the point where you seriously can't cope any more.
- Come out to your parents and work through the consequences
- Contact the various resources I mentioned above and move out as soon as possible, then start your transition.
The longer you wait to deal with dysphoria, the worse it gets. You say that coming out now will ruin your life... but what will
not coming out now do? What is it already doing?
Quote from: Harry_1 on March 26, 2017, 05:53:36 AM
Or to put it more practically, if I pull out the hairs above my lip, are they likely to grow back? Because I really like my mustache hairs 
Most of them will grow back, but it's possible that you might damage the roots of a few hairs whilst plucking them, in which case they won't. So in later life should you choose to grow a moustache, you could wind up with bald patches. So no, it's best not to pull them out.
Oh, and finally... get yourself to your GP (you can do this without your parents' knowledge or permission from age 16) and ask them to refer you to an appropriate NHS GIC. Tell them you do not want your parents to be notified of your appointment or its outcome. If you can, ask them to send you any correspondence by email rather than post so your Mum can't snoop on it. Some GICs have shorter waiting lists than others, so it all depends on where you are in the UK. If you're in England you have a choice of several, and the right to attend any of them no matter where you live.