The Titanium cage won't rust, but I always throw waterless grease on the bolts as water always sneakes in. It won't hurt even a dry weather bike. I use TF2 lubricant with Teflon, which is the stuff.

no in short, long answer - only same metals will seize, usually through surface oxidization, titanium has a naturally smooth surface finish and does not suffer from oxidisation, it does however have non-stick properties, this said i would advise the use of a small amount of threadlok, this will also prevent the ingress of moisture down the threads.

You shouldn't need to, but a little spot on each bolt wouldn't hurt. They can be a little hard to come by, but a set of Ti bolts may make you feel a little safer about any bonding issues.

I haven't used any and had no problem

Yes, you should use some kind of grease, especially if the two metals are different. I use some copper coloured stuff, can't remember what it's called, but anything designed to stop cold welding of parts should do.

it is good practice to grease threads of metal components, especially metals of differing compounds.