hi there - I trained in medicine at a leading London teaching hospital and know some nurses who I have worked with on the wards. Let me give you my own opinion about your queries - hopefully it will help you to hear it from personal firs-hand experience - I spend a little time teaching A level biology with specialism for entry to med school.
1) Of course you are allowed to put whatever course
you decide to on your UCAS application - however, universities will note that nursing is your last choice, so although it might act as an insurance choice, it will appear as if it is not your preferred course, and so will carry less weight.
2) The nursing course is competitive but defo less competitive than is medicine - if you are quite confident about getting into medicine [I presume your A level predictions are at least AAA if not A*AA or higher, in which case, if other things are very good [a brilliant personal statement; with reference to some very impressive work experience and what you learnt from it; a good BMAT and/or UKAT score when you come to it, etc], then [and if you are using nursing as a kind of backup], I would place nursing applic-n as 3rd/4th choice [e.g.] rather than 5th, OR BETTER STILL, NOT AT ALL!! [sorry if this sounds discouraging, but is my own view from a few years experience of getting students entry into medical school]. On the other hand, placing two [in this case not greatly] different courses suggests indecision WHEREAS one of the criteria unis look for for any course is a passion for the subject matter of the course, esp for medicine.
3] Sorry again for being blunt, but interview invitation IS A GOOD NEWS THING and you should not dread them - most interviews are intended to be passed and the deans/admissions officers look for personality and personal strength issues, and to seek reassurance that the applicant is a "normal" person RATHER THAN as a grilling station. Since your main chosen course is medicine, where it is v unlikely that you will get an offer without an interview, you will need to accept this as s-t you cannot "escape", so to speak!
4) If nursing is your chosen career RATHER THAN MEDICINE [perfectly understandable if that is your inclination], then select more university choices for nursing and ONLY one or two for medicine - as medicine has higher requirements than nursing, it makes sense that you should place the medical schools at the top ALTHOUGH UCAS clearly states that these days the order of unis you put on your UCAS form does not necessarily indicate any order of preferences. Although I do not have much first-hand experience of nursing applications, your list of items sounds like a valid list - remember that nursing career requires a v pleasant, impartial, patient, understanding, caring, sympathetic person, who is a good listener IN ADDITION to the academic ability, so these qualities will need to be evident in your PS and at interview.
Best of luck & be safe! - if you need a set of tailored recommendations please feel free to PM me!
M