France - Austria - Italy Rail ticket~

For your schedule, you will travel west to east and east to west for few times. It only waste your money and time. Soi, if possible, you can resechedule to Paris -- Rennes -- Paris -- Rome -- Venice -- Salzburg -- Vienna.
First of all, let see the price of single ticket if you buy it separately in Europe, price will only be higher in Hong Kong as travel agent will charge your service charges. (Mont St. Michen has no train station, you need to go to Rennes then change bus there, return bus fare is around 20.00 euro)

Paris -- Rennes: 55.00 euro (TGV)
Rennes -- Paris: 55.00 euro (TGV)
Paris -- Rome: 100.00 euro (Artesia, 2nd class couchette)
Rome -- Venice: 56.00 euro (Eurostar Italia) / 46.00 (IC, slow train)
Venice -- Salzburg: 50.00 euro (EN, arrive at 04:30, 2nd class seat)
Salzburg -- Vienna: 44.00 euro
Totally, it will cost you 380.00 euro.
If you will go to Mont. St Michel and return to Paris at the same day, you will travel by train on 5 days. Then you can consider to buy a Eurail Selected Pass 3 countries 5 days. The cost of train pass for youth younger than 26 is only 209.00 euro. Adding up those resevation and charges, it should be less than 250.00 euro. It's much much cheaper than buying ticket separately !!!
sidhk_2000 has given you a good advice for reschedule the plan. However the train from Paris to Austria (Salzburg or Vienna) must pass through Germany. If you consider this route, your Eurail Pass must also cover Germany even though you will not stop in Germany, that means you have to buy Eurail Selected Pass 4 countries 5 days... it will be more expensive.
If you have a Rail Pass, you can go to train station to reserve ticket 1 - 2 days before travel (of cos as early as possible), then you will have a big chance to get a seat. Don't worry about full house so much.

I am not answering your question but want to give advice on your plan.
I think you go from Paris to Salzburg, then Vienna, and thenVenice....... is better. You don't need to go to the East and West and East again.

It is depending on how long you will stay in Europe.
I have a similar trip with you except I did not travel to Italy. (Many years ago)
It seem to buy individual ticket to be cheaper.
If you have partner that are also under 25 years old, it must to buy the Euro
discount pass for 2 persons.

You better to buy the ticket in HK because it is cheaper.
You can go to student tavel agency to buy the ticket. Sometimes they have
further 3-6 % discount. Moreover, they can also provide advice to you.

Even today they can provide advise for my business trip.

Enjoy you trip! Good Luck!