First of all, sorry to hear you've had what sounds like quite a nasty injury. It's quite common after a trauma (be it a fall or car accident/injury) for things to take a while to settle, unfortunately.
With regards to your neck, are you doing any exercises or things at home aside from the heat / TENS, or just having acupuncture? Is the acupuncture helping with your pain levels in your neck?
Have you had any x-rays or investigations done? I take it from your post that your back pain has been mainly right-sided and is now coming on on the left? Have you ever had leg symptoms prior to these incidents where your legs gave way? Generally, backs don't "give way" the same way legs do when injured...and with most people, it's the excruciating pain that causes your legs to give way, however, it sounds like there is a neural element with regards to the pattern of symptoms you're describing (alternating left-right sides and buttock/thigh/shin pain).
Are you getting any pins and needles / numbness / tingling in your legs or feet? No numbness or pins and needles around your groin? The leg symptoms to me sound like sciatica (is the pain coming down the back of your thigh?). Sciatica can be due to a number of factors....there is a muscle in the buttock (called piriformis) which runs over the sciatic nerve, so if this is irritated/injured or tight, it can cause sciatica symptoms (pain radiating from your buttock into your leg). Is the buttock pain less after a hot shower/bath?
If you have any concerns, I'd definitely discuss with the physio. Bear in mind, most back pain does settle itself eventually, but it sounds like you've bad luck with this injury (with regards to neck/bursitis and back) so give it some more time and if the pain is agonising, take some painkillers to give you some relief.