Well done for being smart about nutrition, so I am going to answer your questions first and then add a couple more of nutrients you haven't considered.
1, What's the best type of milk alternative to add to tea or coffee and can you add it straight to a boiling drink? I'm sure I read somewhere about some things curdling.
There is so much choice, it really comes down to a matter of taste. Personally I use unsweetened soy milk for teas and coffees, but almond milk for golden latte and matcha latte
2, What's good to pour on your bowl of cereal in the morning? Is there one thing that will work for that and also for adding to hot drinks to avoiding buying two separate products all the time? Would obviously buy two things if one doesn't work for both.
No need to have anything different.
3, I tried a vegan burger the other day and when I looked at a review online it said it didn't contain soy protein as if that was a good thing. Is there something negative about soy I should be aware of?
There isn't anything wrong with soy unless you are taking thyroxine. Soy is actually protective for women with a genetic predisposition to cancer. However there is a big difference between soy and ultra-processed soy based product. I eat tofu and tempeh, but not a single of the soy protein or other industrial franken-food
4, Is there something you should be eating to ensure sufficient calcium intake if you're cutting out milk but worried about bone and teeth health?
Tofu, especially the hard one, is a great source of calcium and dark green vegetables. A PP is right about spinach being a bad source despite a high calcium content as the oxalate content inhibits absorption. Many vegetables milks are enriched, with added calcium so there isn't much difference to you taking a calcium supplements BUT if you don't take vitamin K2 at the same time of your calcium supplements, the calcium will end up in your arteries and not your bones and will actually harm you.
5, Is it a good idea not to eat too much quorn, tofu and other meat alternatives? I don't know if there are any negatives about any of them.
Don't put quorn, an industrial edible mould (the soil mould Fusarium venenatum strain PTA-2684 if you really want to know) and tofu, a food that has existed for more than 1000 years in the same bag.
I don't eat it, never have tried and have no intention to. I eat vegan to improve my health not to eat ultra processed food. I eat tofu, but also beans, lentils, chickpeas and you can make great burgers with chickpea and chickpea flour.
Aim at real food, not some funky invention.
Now to the nutrients.
As already said B12 is a must.
You also need to consider iodine unless you are eating a lot of seaweed. I take iodine drops nonetheless .
A big forgotten one is omega 3. Plant based omega 3 exists, mainly flaxseed, chia seeds and so on . However the conversion from ALA to EPA and DHA is poor in most case, inexistent in some. I tested my omega 3 index, and despite eating more than 800% of plant based omega 3, my levels were 3.8. At 3.5 you have cognitive disorder and dementia. You want to be above 12. So now, I take algae-sourced EPA DHA supplement.
iron is a tricky one, as the plant based iron is a bit harder to keep as having tea or coffee will nullify your iron absorption but vitamin c will boost it.
zinc is hard to reach unless you mindfully go for seeds or even tempeh.
Selenium is another forgotten micronutrients but a single brazil nuts will have you covered.
I use this free website to keep track of my nutrients. cronometer.com Use it for a week or so to understand how to correct your diet.
I would also recommend a vitamin D supplement.
When I first became a vegan, I was very against supplements with the exception of b12. I bought vegan nutrition books, did online courses and I believed to have the perfect diet . Yet blood tests proved me wrong and I was becoming malnourished. Be nutrition conscious but have regular blood test including the omega 3 index.