I wouldn't try it aside from the brands 9sp to 10sp is generally not a good idea.

Im currently using an old 8 speed Dura Ace front mech on a 10 speed Dura Ace set up with STI`s and the mech works perfectly fine. Front mechs are not asked to a lot, its only a change over two rings so trial and error usually brings out a solution

Yes this will. I use this Derailleur with an Ultegra compact chainset.

As long as the left hand lever on your 105`s are double then this will work no problem. The same as it would work with a Dura Ace front mech or a Sora. Its only a two ring change and there is still enough room for the chain throw at the rear. Front mechs dont tend to prove to be a problem when mixing around groupsets etc.

Yes it will work fine. The double shift on your left hand lever covers the space between two chain rings and takes into account chain throw at the rear. The mech therefore will work fine with the Tiagra left hand lever