What course do you want to do at university first of all? Those are some really strict entry requirements.
If you want to do this course then realistically your only option is to continue with National 5 (N5), getting B's in most of your prelims is a good sign though and its likely that if you continue studying as you are now you will pass your final exams.
You are right in saying that your subject choices in S5 and S6 are very important. The requirements do not refer to level A, they say that you have to get either 5 Highers (H) at A grade, or 4 As and 2 Bs. Crashing a higher is where you studying a Higher without completing the relevant National 5 course, in your case crashing Higher Chemistry without taking it at N5. I haven't taken Chemistry at Higher or N5 so I don't know what the course is like so I'll leave it to someone else to answer that one for you.
In order to pass the entry requirements for the course you mentioned, you would need to take 5 Highers in S5, 3 of those would have to be Chemistry, Biology and either Maths or Physics. You would then need to take 2 Advanced Highers (AH) in S6, with no requirements on what you take at AH, but schools generally only allow you to take AH when you have completed the equivalent Higher course, for example you would need Higher Chemistry to do Advanced Higher Chemistry.
You could get away with doing National 4 English this year and then doing National 5 is S6, not ideal because if you did N4 this year you would progress to N5 in S5 however the requirements state that you need to have 5 As or 4 As and 2 Bs all at Higher. You only need National 5 English to enter the course you mentioned.
If you have any more questions then please don't hesitate to ask.
(Sorry if that was quite long)