seat bookings!

hi team

just another question regarding seat bookings. 

we are doing the long journey from Rome to Paris, - and we want to go through southern France instead of Switzerland. I know the train is leaving at 8:20am from Rome - on the 12th May but i wanted to ask, as it stops in Milan and we have to change train stations from Centrale to garibaldi in a half hour gap between the trains it said that the TGV inoui is departing from Garibaldi station in Milan.

 Just looking at worse case scenarios 

if. 

1. we miss our train how can we find if more trains are coming? and is there a chance we can keep our seats on the next train if we do. and
2. can we book family booths through rail plus? ive heard they have some
3. is there any recommendations on the best way to get to garibaldi, although its close walking alone ins 20 minutes, can anyone recommend the quickest way to get to garibaldi station from Centrale which gives us some time to find the platform?

your assistance is greatly appreciated!