I'm not sure how qualified I am to answer this, considering I'm doing Unit 3/4 currently; however, at my school we weren't allowed to do unit 3/4 without doing the unit 1/2. If your school is offering it, then by all means feel free to do it, but I'll outline why our school wouldn't let us and my personal thoughts on how I would feel doing it without the 3/4.
My school wouldn't let us do it without the 1/2, mainly because the sheer amount of metalanguage that you cover within unit 1/2. While some of this is recovered, a lot of it is assumed knowledge; however, if you were willing to spend a far portion of your holidays reading your textbook and creating a metalanguage bank, I'm sure you would have no problems with this aspect.
Another reason is the difference in SAC responses. For the exam, we have three parts: short answer, an analytical commentary (AC) and an essay. Now while these sound very similar to a general English response, a key aspect of these are your use of metalanguage. Metalanguage is essentially just like your business and legal terms, words that are able to succinctly describe aspects of English, such as jargon, which is specialised language from a specific semantic domain (which is another metalanguage term). These are expected to be used throughout your short answer and AC's. Now the short answer are fairly similar to your content subjects; however, I would personally describe an AC as essentially language analysis. You pick out aspects of language, such as sentence structures, sentence types, elevated lexemes, colloquial language and more, and you discuss them in terms of the function of a text, the register, social purpose and cultural context. Without going into too much detail about these (which I may have already done sorry for the lengthy response), you need to be really good with content and memorisation, and applying big ideas and asking "why" they are used. We also have essays, which are fairly similar to your English essays; however, they are not based off books, but rather big ideas surrounding language such as "How does formal language fulfil a variety of social purposes within a contemporary Australian context. Discuss". One thing you may find is similar, is quotes are recommended to use in your responses as they can really tie your points together and can show an additional understanding of the prompt. You will also need to listen to the news a lot, read newspapers and etc to collate ideas surrounding language use within Australia to use within these essays.
You also mentioned time pressure, so I will mention that you have 2 hours to complete the Eng Lang exam, with 15 minutes reading time, and three different sections. This means you will have less than an hour for each essay (I call AC's essays even though you don't write a conclusion), which may not help you in terms of time pressure.
Now I know I'm sounding overly critical, but I really do love Eng Lang. I'm very math-minded, so this is something I enjoy, especially the whole analysing deeper meaning within current discourse. However, I do also realise the sheer amount of work it has taken me to raise my grades, even despite doing unit 1/2. My best suggestion for you would be to consider meeting up with the teacher at your school that is already teaching English Language, discuss with them how much work they would believe it would take for you to catch up, as well as asking them for extra practices to learn the styles necessary for Eng Lang if it's something you're really passionate about. Now this is all really dependent upon the score you're aiming for and the time you're willing to put in, but I do believe that it's something you could definitely succeed in if you're willing to put the time into it.
If you've got any further questions, I'd totally be willing to answer them if I can
