No.
I do - smells lovely!
Yes.
Yes
Yes you can. I would not advise to burn in an indoor fireplace due to chemicals used on the wood and also because of the screws and nails. Also if you decide to burn it never leave it unattended and always burn in a covered fire pit.
It is a bad idea. You will ruin your fireplace, and when the dry little needles/leaves catch fire they will spread everywhere. Learned this the hard way.
While you can burn most woods, I would pass on the arborvitae. As an evergreen, you will get a LOT of creosote, fast burning, short lasting fires. It also contains a toxin believed to be a hallucinogen. A small amount used as kindling may not be a problem.
If you mean a residential gas fireplace, it would burn natural gas, which is mostly methane (CH4).
no
Yes it can
Check with Martin for an owner's manual. Unless a fireplace is specifically rated as multi-fuel, a gas fireplace cannot safely burn wood.
No, it is not safe to burn. It is toxic.
yes you can burn cactuse