Congratulations on finally getting your honeymoon!
If you need to be in Monterosso on 9 October, it's certainly possible but will require a full travel day. I find that long train journeys are sometimes necessary, as they do eventually get me to the desired location, and in some cases it's necessary to crack on and get it over with. This is the route I'd probably use.....
- Depart Avignon TGV at 10:02, arrive Nice Ville at 12:55 (via TGV high speed, which travels at up to 300 km/h).
- Depart Nice Ville at 14:09, arrive Genova Piazza Principe at 17:05
- Depart Genova at 17:47, arrive Monterosso at 19:03.
The trains suggested above are one of the fastest solutions, with the fewest changes.
Each of those trains has compulsory seat reservations, which will be provided with your tickets. If you wish to pre-purchase your rail tickets, you can do so at www.capitainetrain.com (registration on the site required). If purchased at up to 120 days in advance, significant savings are possible on the Italian trains if you're able to get Super Economy or Economy tickets. However, note that Super Economy are non-changeable and non-refundable so choose carefully!
If you haven't used trains in Italy before, a few points to note.....
- If using slower Regionale trains, tickets MUST be validated prior to boarding on the day of travel, or you'll risk hefty fines which will be collected on the spot!
- If using faster InterCity or Freccia trains, tickets don't have to be validated but are specific to train, date and departure time, and can ONLY be used on the one train listed on the ticket. If you board the wrong train, hefty fines!
If you need any directions on getting to your hotel in Monterosso, post another note.