You might want to consider twice, because it is not 100% sure that they will not ask more documentation than NON O for your DTV extension. If your treatment plan is longer than a year and you have enough evidences to prove it i would say let's go. But if you do not have i would stay NON O
Well, it is better to write your request there as this could help the community and this is the goal of this group. If you're loking for personal advices regarding your specific situation, you can consider hire an agent but in my opinion, it is useless. you do not need one.
depend of the embassy you apply. They have different requirments. Kuala lumpur, taipei are known to be more difficult, Hanoi, HCMC, friendlier options.
sure it is cheaper than return to UK. The embassy requested documents translated in English but they didn t ask for anything notarized or legalized. I provided my documentation in original + translation merged all together