1. Yes you can use no offset with the frequencies. All frequencies can be ran in any mode you choose provided it fits within the abilities of the hardware. The positive offset is just a recommended starting point.
The only true way to find out if they will be effective is to try all variations and keep good notes and make detailed observations. Not everyone responds the same. Generalizations can be made of course if you have enough data.
I've tried many a configuration, and have my reasons for using one vs. other depending on what I"m after, now that I've had significant time trying various protocols.
Over time you too will build up your preferences. If you are not sure where to start, the recommendations were there to give you that starting point.
2. Take a look at this thread:
specifically the response by David Bourke.
"Why did I and four other people burn holes in our skin from using 100% Offsets in Contact Mode (as you recommend) using HC or KHZ sets for longer than seven minutes?"
You may try the 100% positive offset, but you must keep treatment times under 7 minutes for contact. If you want to use this for longer times, use 0 offset.
For more details, please check the link: