I am thinking of taking this itinerary on a Sunday next summer. Itinerary listed below:

Eurostar #9016
10:01 London St. Pancras Int’l
13:17 Paris Nord
Transfer 1 hour 10 minutes
TGV #9249
14:27 Paris Lyon
21:50 Milano Porta Garibaldi

Here are my questions:
1. Where is the best online portal to buy tickets? (I saw this itin on LOCO2)
2. Eurostar Standard class luggage storage space, any issue with one 22” roll-on-board and a backpack?
3. I am concern about the transfer time (1 hour 10 minutes) between Paris Nord and Paris Lyon. What is the best (least time) mode of transportation, taxi, uber or metro?
4. What if my Eurostar train was late, and can’t make it to the TGV, what happens to my ticket and onward journey? (one time I experienced a 3 hour delay between London and Paris Nord)
5. Arrival late at night at Milano PG, any security and personal safety concern?

Thank you in advance for your advice and suggestions.

PS: I enjoy riding trains, hence ruled out flying.