Have you seen a GP? Remember it's a medical condition primarily & prescription products / antibiotics may well be needed.
Okay -what's helped me (bearing in mind different things seem to work for different people): finacea gel (azelaic acid) on prescription - helps with everything for me - spots, flakiness, redness. I tried metrogel & metrocream first - GPs often seem keener to prescribe those - but they were hopeless. Finacea can cause itching & stinging to start with, but I'm so glad I persevered with it.
General skincare: I use a milky cleanser in a range called Purity Organics, that used to be available in Superdrug, but I now have to order. My skin dislikes lots of natural products - inc. rosehip oil & tea tree, as you mentioned - but this is okay for me. Used a micellar cleanser - can't remember the brand - a couple of years ago that was also okay.
I use Simple moisturiser. I know lots of people can't stand it but it helps me. When things are really sensitive / dry, Avene skin recovery cream is better, or I mix them depending upon the condition of my skin, which veers between oily & dry. Always sensitive though.
It's extra important for anyone with rosacea to protect their skin well from the sun. I couldn't be without a physical sunblock - can't cope with chemical filters & they set off my eyes (have ocular rosacea too). I use Elta MD UV Physical SPF 41. Have to order from the US. It's lightly tinted & I use it in lieu of foundation most of the time. Have used Clinique Redness Relief foundation too, which is good, but irritates my eyes a bit. And Susan Posnick Colourflo mineral makeup doesn't irritate me.
When things have been at their most inflamed I have had courses of oral antibiotics. Am allergic to tetracyclines so have used clarithromycin, which has been shown to work as well as doxycycline & with far fewer side-effects. The regime that was tested was 500mg daily for a month, then another month on 250mg daily. That worked for me when nothing else did & when the state of my skin was making me really fed-up. Haven't needed antibiotics for about 3 years now.
In terms of causes, I don't think there is one cause. For some people, digestive issues seem to be a factor & changes in diet help. They make absolutely no difference to me. Other people are flushers / blushers from childhood, & it gets worse with age - I'm guessing they just have ultra-sensitive facial vasculature.
There's evidence of a link with excessive numbers of demodex mites, as I'm sure you know, especially in cases where spots & sudden onset are factors. Demodex are killed by tea tree oil, but obviously that's irritating. There have been some studies on the use of topical / oral insecticides, like ivermectin / permethrin, but I don't know how conclusive they've been.
I read research a while ago about people with rosacea having abnormal cathelicidins - a type of antimicrobial peptide - in their skins. These produce a huge range of inflammatory symptoms. Still begs the question of why some people develop these in an abnormal form & others don't.