Oh dear, Spider, this sounds really miserable .
Jelly is good for hydration as it's virtually all water. Plus the sweetness might make encourage her to take some. Make sure you use the sugar version, though, not the sugar-free.
What about pureed avocado or banana? Soft, sweet, nutritious and calorific. I would imagine that, with all those wounds in her mouth, fruit might be too acid and sting.
When you put anything in her mouth, does it help to try and aim it into her cheek, rather than onto her tongue?
I find that the 'conventional' syringes you get with medicines are the wrong shape for the baby's mouth and they rarely co-operate with it when they are very young and if they are feeling particularly poorly. The Baby Nurofen syringe is a better shape, but evn that has a square end and requires the baby to open wide. The best I've found are the eye-dropper style, because they are smooth and rounded at the end, and bcause you squeeze the bulb, rather than push a plunger, you have greater control over it.
Even if you manage to squirt a just couple of ml of liquid into the side of her mouth every 5mins, you will go a long way towards preventing dehydration. You can even do this while she's asleep as long as you only squirt in 1 or 2 ml at a time. My mum said she kept painting my brother's lips with water using a soft paintbrush to keep him taking some liquids.
Hope this eases soon. It's so gutting to see your LO ill and distressed, and not be able to make it all better.