How do I pronounce romanised Chinese names which lack tone marks?
As a native speaker, this is what I do in such a case:


If my listener is not Chinese, does not know Chinese, or I am speaking in an event that doesn't require my listener(s) to know Chinese - I pronounce it in whatever tone I feel comfortable. Sometimes I mimic the listeners' pronunciation. (However, if I can guess the tones, I may tend to guess, because that makes me comfortable.)
If my listener is Chinese, I usually guess the tones, and add that I am not sure if the tones I used are correct.


Chinese people will understand that you cannot make the tones perfectly correct - when you are only given the Pinyin without tone marks - because there's missing information.



Here is my idea about your options:


Using neutral tone is creative :-) I have never heard anyone do that. It will make the pronunciation funny.
I believe this would be okay if both you and your audience are English speakers, corresponding to my first point above.
This is corresponding to my second point above. Guessing the tones require knowledge about common names and naming conventions. This is possible, but may not be accurate for all names. This would be the best you can do. In most cases, last names are easier because there are only tens of frequently used last names.




A few examples for guessing last names: Zhang (first tone: 张, 章), Li (third tone: 李), Liu (second tone, 刘）, Wang (second tone, 王 - but it might be first tone, 汪, which is less frequent).

First names are harder. For common names it is still possible, like Ming (second tone, 明), Hao (forth tone, 浩 or 昊). Names like Jinping would be difficult but I would guess it to be 金平 if it is not the president's name, because 金 is more common than 近.