Paris + Northern Italy 14 days

Since the bulk of your question deals with Italy you would probably be better off posting over in the Italian board but here goes.

Both Trenitalia and SNCF are advertising 7 hour direct trains from Milan to Paris again. It's been a while due to tunnel work but as of May they are offering this direct route again as of May. (It's too early to actually book August yet.) There are few direct routes but also routes with one change - usually in Basel - are available at 8:00-9:00 hours in duration. Unless you are going to make an entire day of it you should probably plan to be in Milan the day before and take a morning train to Paris.

You could also fly - many cities have flights to Paris - but the budget internal airlines usually have severe bad restrictions between them and name brand carriers like Air France you will probably pay a large amount to fly four people. Check Pisa and Florence and also maybe Venice and Milan as outliers for flight pricing. Paris Orly is much easier and cheaper into the city than CDG.

If you have 14 nights you can plan from there:
4 night Venice
3 nights Florence
2 nights Siena
1 night (at least) Milan for train or another city for flight
4 nights Pairs

You should look to fly into Italy - probably Venice - and then out of Paris back home.

Venice and Paris are two of my favorite cities but I would be one of those people to tell you not combine them - but here we are.

Have a great trip,
=Tod