I can't answer the first part of your question but regarding manual work on a converted machine the answer is a qualified yes.

The machine axes can usually be moved using a manual data input option (mdi).

You can add a manual control with a manual pulse generator (mpg).

With some setups you can add manual handles to the axis drive motor shafts to maintain full manual control of motion. This is the option that will get various opinions as to whether it is a good idea or not. There is some risk that manually driving some motors will generate electrical currents that will damage the motor drive electronics. Work around for this include disconnecting motor leads to use the manual handles.

I have put a description of one way of doing manual work on a CNCed mill in my album CNC MDI job. This may help you to see that there is some middle ground between full blown CNC and completely manual machining.

Martin

http://www.model-engineer.co.uk/albums/member_album.asp?a=42009

Edited By Martin Connelly on 25/01/2017 19:14:38

I have converted a few manual machines to CNC (Chinese multi-machine, Harrison Miller, Holbrook Minor lathe).

On the Chinese Machine and the Harrison I went to some lengths to retain manual operation.

(A coupling for a handle on the Y axis, the option to un-clamp the screws and rotate them with handles on the X and D, both of which used rotating-nut on the ballscrews). On the Holbrook lathe I didn't bother, having noticed that I had _never_ felt the requirement to use the other machines in manual mode.