1. If a farmer uses fertilisers that contain animal products, can the vegetables grown still be considered vegan?
I think this is a point to be considered only by the very strictest vegans. There is veganic produce. However, the vegan police aren't going to come and get you because blood or bone was used in fertilizer of produce that you purchased. In nature, dead bodies and feces are the natural fertilizer. It's just that in nature animals aren't systematically enslaved and slaughtered for their flesh by humans. Manure, specifically, doesn't seem to have any ethical properties as far as I can tell. Using poop as fertilizer isn't an indication of exploitation or murder, unless you're some kind of fundamentalist, because you could use poop from animals in a sanctuary to grow vegetables. They're going to poop anyway.
2. Can I use compost to fertilise plants if worms have defecated in the compost?
Yes. If you don't, the environmental police will come to get you.
3. Can I still perform fellatio on my boyfriend? Sorry for the TMI, but genuinely curious (or at least he is, i'd love an excuse to stop).
If you would love an excuse to stop fellatio, then you should stop. In any relationship you shouldn't have to perform any sexual act you aren't comfortable with. This is called consent culture. The fact that this bothers you so much that you would express this on a public forum indicates to me that your partner might not be respecting your personal sexual boundaries. You don't "owe" him any particular sex act (and vice versa). This is why marital rape is a thing now. Not saying that he's raping you, let me just clarify that, but the idea that it's your "duty" to perform oral or anal or bondage or anything you feel bad or uncomfortable doing is wrong. Even if it just means you perform fellatio without swallowing, or even if you don't do it at all.
That being said, any consensual sex act is vegan, because consent is present.
4. If I taste a meat product but don't eat it, like if i just lick it, is that still okay?
I don't even understand this question, I mean why would you do that, but yes it's still wrong because it involves animal exploitation, which is why vegans also don't wear fur, leather, or cosmetics tested on animals. You don't have to eat something for it to be exploitation.
5. How can you keep a balanced diet? Like get enough protein, iron etc.
Absolutely, yes! Protein is super easy to get from eating a wide variety of foods and getting enough calories. There's very little risk of getting a protein deficiency unless you literally starve yourself. If you crave a high protein diet for any reason, you can stock up on tofu, beans, nuts, nut butters, seeds, pulses, vegan milks, and veggie meats if that makes you feel "satisfied." You DO NOT have to engage in any particular diet to be vegan. You can do High Carb Low Fat, you can do oil-free WFPB, you can do high protein, you can do a mix of whole foods and junk foods...just make sure to eat enough calories, a wide variety of foods, and not to get ALL of your calories from junk foods or processed foods. Even a "junk food" vegan needs some fresh fruit and veggies, and some beans once and awhile. Make sure you take B12. Taking a multi-vitamin or a DHA supplement is a personal choice. If you know you have a hard time getting a variety of foods, or eat a lot of junk food, or just don't know how to eat yet as a vegan, you might want to get a multi-vitamin, or give it to kids if you have any. You can get Omega 3s from walnuts, flax seeds, and hemp, or by occasionally taking an algal oil supplement.