Not sure about arriving overland for Chinese nationals, but I know that Chinese nationals can get a 15-day on arrival visa for 1000 Baht at airports only I believe. Therefore, it may be necessary to get a 60-day tourist visa from the consulate in Kunming if your girlfriend wants to stay longer than 15 days since those visas are non-extendable I believe. I don't think it is really necessary to produce a return or round-trip air ticket but each consulate has its own rules, and for Chinese nationals they will probably enforce such rules more rigorously than for other foreigners.
A return bus ticket is probably not of much use, as I never heard of this being accepted or even heard of anyone purchasing a cross-border return bus ticket, I think it has to be a plane ticket - but do check to confirm.
China Eastern has 3 weekly flights from Kunming to Chiang Mai. Otherwise, Thai flies 5 times weekly from Kunming to Bangkok with connections to Chiang Mai.
The Burmese do allow vehicles into their country - but mainly from Mae Sai in Thailand. I have asked about this and it is indeed possible. Even non-Thai foreigners have managed this, but driving a Thai vehicle of course. It does require lots of permits and money to pay for these permits along the way aswell, but I have seen a personal account of a foreigner who did this - I can't remember the website, but it's easily searchable by googling it. There are restrictions and you can't go everywhere you want either. I think it may only be possible to drive from Tachilek (opposite Mae Sai in Shan State) to near the Chinese border and back.
From China, the conditions and restrictions may be very different. Myanmar is a fascinating but also very strange country with some of the most bizzare laws in the world. You can definately drive your own Chinese car to the border at Ruili but as for progressing further into Myanmar you might want to enquire in Kunming or Ruili if anyone can help you further.
I think it's unlikely, or maybe you'll be permitted to take the car to the town across the border (Mu-Se or something I think it's called) but no further, since that town is just an extension of China anyway (Chinese is spoken, most people are Chinese and Yuan are used). Then again, just being able to spend a day driving in a border town is probably not what you want so do enquire somewhere official that may be able to help you further as I can only comment on driving in Myanmar from Thailand.
Do you mean that Laos used to drive on the left hand side of the road like Thailand? I don't think so. Laos has always driven on the right due to the French colonizing their country until 1945 although they officially gained independence in 1954 I believe it was.
Myanmar (Burma) used to drive on the left until 1970 until a fortune teller told the then leader to switch to the right to improve his karma or something. You can do a search on the internet and you can easily confirm what I just said.
You are right about Laos having many right-hand drive vehicles though. Many, if not most of these are from Thailand, but since Laos has legitimate car showrooms now, there's little reason to buy a Lao-registered right hand drive vehicle unless a particular model is not available in Laos these days.
I have stopped in many Lao villages between Vientiane and Vang Vieng along the way (driving a Lao vehicle) and they were a pleasure to pass through. People were friendly and helpful. Mind you, the only reason to stop would be for petrol/gasoline, a toilet break or buying some snacks to eat or stopping for a meal.
Don't drive at night because there's no lighting, you can't see the stray animals/kids/grandmas and grandpas etc. on the roads, dangerous overtaking/passing is prevelent and many vehicles drive without their lights on. No service stations would even be open at night either (all gas/petrol stations even in Vientiane close by 7 or 8pm, in the countryside possibly even earlier). Also, no sane local would drive at night outside the cities and yes banditry may be an issue on some routes though I think things are considerably safer these days. I would say the risk of driving at night comes more from the fact its extremely dangerous in terms of all the things described above - don't do it, just get up early and start driving again during the morning and you'll be fine.
My post may come a bit late since I just saw your post now, but for future reference, stay at one of the many hotel apartments that with signs in English and/or Thai as follows: Daily/Weekly/Monthly รายวัน/รายสัปดาห์/รายเดือน since they offer the best rates. Hotels away from the city are better value and may offer something similar.
If you want to save money stay away from touristy places such as Sukhumwit (with Sois numbering 40 or less) or Rajdamri etc. as they are expensive. Yes, you may have to sit in a cab for a while if you stay further away from the city, but if you can find a place to stay near an expressway on-ramp you're often not going to spend much more time traveling than if you were staying somewhere like Sukhumvit soi 11 as the above poster has suggested, even if distance wise it's much further but given the traffic in Bangkok I'd much rather stay further away and you'll probably save both money and yes, surprisingly even time....believe me, I know Bangkok as I've lived there for a while and I know that traveling 30km can sometimes be quicker than traveling just 3km depending on where you are in the city.
Above advice is fine, although I thought I saw Yuan on the noticeboards on the moneychangers in Vientiane (just check and if they don't accept them, find another moneychanger or guesthouse or internet cafe as above posters have suggested). I've changed numerous currencies in Vientiane before, but Yuan was never one of them since I'd never previously visited China before now.