Unfortunately, I can't give much guidance regarding the linguistics side of your question. However, as someone who has graduated from Uni I can give (hopefully) some insight into your worries about doing a 4 year degree. For most people, university is some of the best years of your life and they would love doing a degree for four years! Lots of people who go to uni also end up enrolling into an extra year by doing a Master's, year abroad or year in industry (including myself!), and so end up doing 4/5 years anyway. In terms of 'being a year behind' for employers, I wouldn't worry about this at all. Lots of people don't get a job straight out of uni, and so it's perfectly normal for employers to look at people from all ages - and you'd still be young when you graduate anyway! In my opinion, you'd gain a year of amazing life experiences rather than feeling like you'd 'lost a year'!
Especially with the uncertainty of COVID, I'd be really encouraging of a 4th year.
Doing your EPQ on linguistics is an excellent step! I did my EPQ to help with my application and this really helped. I did a whole paragraph in my uni app on it. If you can, do as well as possible in this because some unis might consider a lower offer for you if you have an A/A* in your EPQ. You might want to research if this is applicable for any unis you are applying to. This policy was very useful for me, I got 2 grades lower than I was supposed to, but I was still allowed in to law school because I did well in my EPQ.
In terms of worrying about whether you'd get in with certain grades, I was really worried about this too when I was looking at applying. I found my dream uni but didn't know whether I would get the grades. My careers adviser told me that there wasn't any harm in taking the chance and applying, and if I got rejected I had my 4 other options or could try again the year after. I took her advice and ended up at the uni! As cheesy as it sounds, you miss every chance you don't take! If you would regret not trying for Edinburgh and are willing to work really hard to up your chances, then I would say to go for it! You have 4 other ucas options to fall back on, or could even take a year out and wait again.
Best of luck for your apps, and don't worry to much