Can I use portable LPG heater indoors???

There are LPG heaters that are made for indoor use. However it against laws in every state to have an LPG tank in a residential building and your fire insurance would be voided if you did have a fire. You would need to have your local propane company install a tank outside and have a plumber run the piping to the unit. They are terrific units when used for single room heating and allow you to turn down the heat in the rest of the house and keep your favorite room toasty warm. Go to Lowe's and look at the ventless wall units and the free standing ventless gas fireplaces.

Not a good idea. The flame will take the oxygen out of the air and you could die of carbon monoxide poisoning.

probably not, since you will need some good air circulation, which would defeat the whole purpose of using it indoors

well ,its kind of like using your bbq inside,i wouldnt and if its a rental property definately not ( most tenancy agreements now days dont allow it)