Read your post this morning but didn't have time to post then & can't believe no ones posted in the meantime.
So two problems, behaviour and writing.
First off behaviour, or rather struggling with managing her feeling and perhaps expressing her needs (leading to the explosions). Get some books (there are some really good ones), and start communicating with her about her feelings, and explaining what feeling are. Help her to identify her feelings e.g. you're sounding sad/cross/happy - this will validate her feelings. Remember feelings are ok, even anger, we all get them, it's how we deal with our feelings that matter and feelings pass, they come and WILL go.
One of the best ways to manage feelings, especially anger, fear and sadness, is through relaxed breathing. I've used the following technique with DD for years now - hold up fingers wiggling them, explain that these are birthday candles and need blowing out with long slow breathes, drop various fingers intermittently in response to them being blown on & repeat until DC has more relaxed breathing (I've seen GP's & nurses faces light up watching this technique)!
You could also try getting your DD to pretend she's blowing up an imaginary balloon, again with long slow breathes, say she is blowing all of the angry feeling into a (red) balloon, when it's full she let's it go and you then make the silly noise as you both imagine it rushing around - the sillier the better.
Also simply getting her to put her hands on her lower tummy and focussing on her breathing. Of course you need to explain how this works and practice the exercises first when she's not feeling angry, so that she will perhaps go along with you in the future when her emotions rise. Ideally you need to get her to use these techniques before a total meltdown, so understanding her triggers and noticing her warnings signs will help.
Practicing a martial art is useful for managing emotions, the discipline and confidence gained can make a huge difference, the same would apply for kiddie yoga, but if you're really rural then those might not be options.
Talk to the school about her having a safe place to go if she starts to feel overwhelmed (10 minutes in the reading corner can be all that's needed) and it's surely better if she gets to use her 'flight' rather than 'fight' response to stress.
As much as you can, try and model this behaviour yourself - even if it's just to say "I feel frustrated so I'm taking 5 minutes time out" (this is easier said than done of course but if your DD see's you doing it yourself it really will help her see it's possible, not to mention let her know that her feelings are normal).
I hope that's helpful - I'm sure it won't be everyone's cup of tea but all these things have helped my DD. Oh yes and every time she manages her feelings, even if it's after 10 minutes of ranting, then praise, praise, praise - well done you're relaxing your breathing or whatever.
The writing just needs little and often and colouring in and drawing, dot to dot, play doh etc. all help to build the muscles and fine motor control and it is better that she colours / draws happily if writing is leading to battles.
Equally keep trying every so often - you could try making story books together, she does the picture and first word and then dictates the story for you to write (little stories one sentence a page type of thing), & you gradually increase the amount of words she writes herself. This will get her creating stories, letters whatever and when her writing catches up she'll fly. Plus you are modelling to her that writing is fun and has a benefit (lovely books that you can read / share with her, which are her very own stories - she's an author)!
If you can manage to squeeze in a few play dates so she can strengthen her friendships too then so much the better.
I realise I've bombarded you and it may not be practical to do everything (or perhaps nothing I've suggested may appeal) but good luck &
- with you looking out for her I'm sure you can help her find her confidence and stay focussed on the joys of the world around her.