Is it possible for a first-time DTV holder to leave Thailand through the Chiang Khan border before 180 days?

No issues border bouncing at Chiang Khong. If the immigration officer on the way out tells you you must stay a day or two, just say okay okay. Stamp in and out of Laos and return to Thailand. The Thai officer stamping you out is not the same one stamping you back in.

Be careful with land border crossing on the DTV. I’ve read & talked to expats who experienced immigration issues upon re-entry. Flight in/out is the best option to ensure no hassle with DTV re-entry. I fly from Don Mueang airport in Bangkok to ASEAN countries without issue on the DTV.

My understanding, Chiang Khan should be OK for DTV.

Cheapest way of flying to Laos is to fly to Udon Thani (Air Asia direct) and get minibus (45-60 mins) over the border. Flying to Hat Yai (Air Asia) and going over to Malaysia by land is also possible.

Only direct flight to Laos is to Luang Prabang with Lao Airlines. It's very close but not typically cheap. There are direct flights to several other places, Kuala Lumpur with Air Asia is often one of the cheapest and nationalities don't need a visa for Malaysia so 2,000B saving there over Laos. China can also be cheap, Xishuangbanna or Kunming (Ruili) as little as an hour, I went to Xishuangbanna for a couple of nights. Many countries visa free to China now too. There are also direct flights to Hanoi (Air Asia) and Hong Kong (HK Express) that can be inexpensive.