Not worth trying to 'do' Rome in a day especially given 9 hours travel time by Freccia trains. It needs loads of time to get to understand as well as visit. Spanish Steps are completely underwhelming and not worth the time given the many, many other sites in easy distance.
Trieste is easily done by train. I made a day trip with my Italian friend from Venice although admittedly, any city needs much more time, but we weren't rushed and it has a very different feel to it having been annexed to Austria. Miramare is also easily visited on the same day by bus.
More likely in Rome or Naples. Not going to happen in Venice.
Whereas Burano and Murano are the obvious tourist stops maybe conside Torcello. Nothing obviously touristy but this is reputedly the origin of Venice. There are few residents but it has a pleasant bucolic atmosphere and a huge basilica with ancient frescoes inside. Padova is highly recommended just 30 minutes by train from Venice. There is the magnificent space of Prato delle Valle which really is lovely. S.Antonio's basilica is worth a visit with soaring pillars and beautiful ceiling apart from anything else that might catch your eye. They say no photos but you will see plenty doing so as I did myself. The star of Padova (Padua) is the cappella degli Scrovegni about a 10 minute walk from the station. The whole of the inside is painted with frescoes depicting the life of Christ and the work was done just over 800 years ago by the genius Giotto. Only 25 visitors at a time permitted fora timed entry. You must book your time and turn up before as late entry not permittd but it is well worth the effort. Photography is permitted without flash or tripods. The frescoes are stunning and there is a guide/caretaker who will explain if needed(as long as you speak Italian) but it isn't really necessary. In Venice ascending the campanile outside St.Mark's is worthwhile for the vistas across the lagoon to Lido, the rooftops of Venice and the Dolomites. If yoou have time, the lovely fishing port of Chioggia is like a mini Venice but much quieter, picturesque and superb fish restaurants. It takes about an hour by bus but there is also a ferry.
The third photo down is such an iconic shot!
Venice rewards a lot of pre-reading to get the very best from it. I have been going for the past 8 years and still finding new delights. Florence is really good but left in the shade by La Serenissima. In those 8 years I have never yet seen the Alta Acqua but warning is given before it happens which is several days a year and not every high tide. There are two train companies - Trenitalia (the State owned company) and Italotreno which certainly runs excellent high speed trains to Florence. No need for first class as the standard seats are very comfortable.
The trains and buses are easy to negotiate. Buy a ticket for local buses in small local shops. Usually it is a prepaid card with a set number of journeys. You validate it as you enter the bus. Buy train tickets at the station from the ticket office as you would at home. You need to mark the train ticket before you board the train at one of the machines on the platform.
I've never had a problem with film either
Mind you, if you are really serious about the art of photography and ultimate quality you would be using a 10x8 camera. You won't blend into the back ground with that whether black, blue or skyblue pink!
It isn't actually the rain that causes flooding but the 'acqua alta'. It only affects the city for a few hours each tide at a few times each year. However, Venice has a quality of light in the latter months of each year that give something special to photographs. Rain also gives a different aspect to photographs and I made some very special photos in Padova in very heavy rain.
I visit Venice regularly and normally use a D7000 or Fuji X-Pro1 and a Nikon V1 as a 'carry everywhere'. With the Nikon, I only use the 'kit' 18-105 lens or, with the Fuji, the 18-55. Venice is compact and really doesn't need long lenses. On a visit to Rome I used the D7000 again with the kit lens and never found it wanting.
Another highly rated repairer is Cameraworks-UK.com. The will CLA/repair/reskin any Leica to an exceptional standard. I have a Leica llf with them at the moment.
Suggest he goes for ultimate quality and move to 8x10 size film cameras. This will soon use that money burning a hole!
You will find that your 24-70 will be perfect. The camera is good enough that selective enlargements can be used when you need something to appear a little closer. You really don't need to be changing lenses in these places. I usually manage my annual Venice pilgrimage with just my Fujinon 18-55 on the X-Pro1.
It depends what you want to do with your photos. If it is simply to enjoy what you take, use whatever you want. Think of it as not 'downgrading' but changing systems. For most purposes I doubt you or anyone else will be able to tell (or care) about any minor differences.