secondly, european rail travel - yes, if you were planning on spending the day in paris anyway, then there is no reason why you shouldn't buy your eurostar tickets now and get the overnight train tickets if they ever exist.
i would warn you though that trains on saturday nights (and poss friday nights i would imagine) to milan may sell out (only an issue if the rome train does not materialise, if it does then as long as you can't buy tickets then no one else can either...) so consider arriving in paris on a day other than saturday, unless you don't mind spending a night there if you don't manage to get your train tickets for whatever reason. given that your holiday is less than 4 months away and so the tickets have already been on sale for a few days/weeks (depending on when you want to travel) the eurostar will probably be cheaper if you don't travel on a saturday anyway.
yes, we have always booked our tickets ourselves - previously we booked the overnight tickets through raileurope.co.uk but now that thello have taken over this is no longer possible. we are going to italy at half term though and so used the new thello site to book that - it was all in french, but not such difficult french that you couldn't book. we did need to enter our passport numbers though, which we've never had to do before.
we don't need accommodation i'm afraid, because the PILs have an apartment in tuscany, so we always go there (oh the hardship...). the seat 61 website recommends a hotel booking website i think, we normally use booking.com if we need one, but that's not the one he mentions.
we normally use 65 litre rucksacks, because we haven't realised that we've grown up yet. also, we don't own a wheeled case. and the DDs are only 6 and 3, so not able to carry/drag their own stuff without it causing more trouble than it's worth. but this half term will be the first time we've travelled to europe in a long time without a pushchair. it has always been fine - we used to have a rule that the pushchair didn't go on the escalators, but once we got rid of that rule any paris transfer was a lot less labour intensive. there are lifts and escalators on the metro, although like on the tube if you are just crossing a railway line there are a few steps up then a few steps back down again. i can report back more fully after half term on the specific route, if you can wait that long...
previously when we've caught the train from gare de bercy, but one of the changes is that it now goes from gare de lyon, i haven't yet worked out what difference this might make to our journey across paris - we normally get the metro, but i think the RER goes straight to gare de lyon so we might get that, although it is a slightly more intimidating experience...
presumably gare de lyon has left luggage, gare de bercy didn't.
don't know if you've noticed, but this is one of my favourite subjects.