chestnut (19/03/2010)
The IPM is invested by the WM with a PM jewel (square with 47th proposition) and collar with silver stripe. He may also be given by the Lodge a PM collar, jewel and breast jewel for services rendered.
So as the IPM he will wear his PM collar and as the acting IPM a PM collar as well. Therefore 2. I think there is a little confusion about what the IPM is. Under normal circumstances (ie not recycling PMs) the Immediate Past Master is, in reality, just the last Master of the Lodge AKA the newest Past Master of the Lodge. He has done his year in the Chair and is then Invested with a PM's (not IPM) collar and jewel because he is now a Past Master of the Lodge.
He is not an Officer and he only sits next to the new WM by virtue of the fact that his memory of the Ceremonies should be fresh enough to enable him to assist the new WM.
He does not get two collars, it is just the one and if he goes through the Chair again (as I am at the moment) he does not get another, he will continue to wear the one that he already has, even though he will once again (technically at least) be the Lodge's IPM he is already a Past Master.
The PM jewel (and sometimes his PM's Collar) will usually be engraved to commemorate his time in the Chair, as it is a present from the Lodge to a Past Master, it does not need to be worn to show his rank that is what the PM's Collar is for.
I hope this helps.