Travel From Milan to Venice

I would stay 3 nights in Venice and only one in Milan (just before flying home). But that is just my opinion.
You can buy online ahead of time, up to 120 days prior to the travel day, if you want a discounted ticket (which sell out fast). However discounted tickets are non refundable if you miss the train or if you change your plans later, so you need to be aware of that.
If you are not interested in locking yourself into a specific time and date, then you can buy the tickets all the way to Venice at the Stresa station, when you depart or a day or two earlier. Trains rarely if ever sell out, so you can always find tickets at the base (full) price.
Use www.trenitalia.com for schedules and prices.
Enter a date in the next seven days to see options (regional trains schedules are loaded only one week ahead, and the portion from Stresa to Milan will likely be on a regional train.

As to checking baggage, there is a luggage check at Milan, but it will be quite costly yo check your bags for several nights. Here is the info. Do you have so much luggage it's difficult to take it all with you? I would caution you to lighten your load to a 21" rollaboard and a daypack. I don't want to be preachy, but you will enjoy your travels much more with less stuff.

If you know your dates of travel, I woudl buy the ticket Stresa to MIlan and Milan to Venice as soon as the tickets are release, 120 days out. When you buy a regionale train (Stresa to Milano) at the same time as a freccia (Milano to Venezia) you can book the whole trip. You can also book Venezia to Milano at the same time, or as a separate trip. There are generally no additional discounts for RT.

Getting to the Daniele is easy in a vaporetto. Do you have Rick Steves' Venice guide? He explains the transportation optioins - including a private water taxi - very well.

For your other questions: Luggage storage is available at Milano Centrale, if a bit pricey at 10 eu/bag/day plus initial 6 eu.
http://www.milanocentrale.it/en/service/detail/left-luggage1
To get to the Danielli, take the No 1 vaporetto from the train station to San Zaccaria. The Danielli is right there on the water front. In fact, one of the three vaporetto docks there is named the "Danielli" dock. That will give you a tour of the entire Grand Canal. Get a multi day vaporetto pass at the Hellovenezia booth to get your return and use on your visit, especially is you want to run over to Murano or other islands in the lagoon.

I agree with Roberto and would encourage you to spend more nights in Venice and only the final one, if necessary, in Milano. There is so much to see, do and experience in Venice, including most of a day out on Murano, Burano and the other islands in the lagoon. Milano doesn't do much for me, although others enjoy it more. If you have an early morning flight from Malpensa, you might consider staying at an airport right by the airport, with a free shuttle to the terminals. I had a good, basic, affordable night at First Hotel Malpensa last October. For your baggage, here's a little tip which might save you a little money. For my most recent trip, I traveled light but knew I would make some purchases along the way and would want to check a bag going home. I ended up buying a very good, light as nothing, totally foldable duffle at a Samsonite outlet on sale at a great price. It was considerably larger than I really wanted but everything else about it was right. Once I got to Italy, I realized that it was so big that I could put my whole carry-on inside, plus some other things like clothing, shoes, a small box of books, etc. While not suitable for carrying around, a few times that allowed me to consolidate my stuff and save some money. It turned two bags into one for checking at a train station, and also made it easier to check everything for a short domestic flight, since the ridiculously low carry-on allowance of 8 kilos total would clearly not allow me to carry on all my stuff. Consolidating your baggage like that might make your plan to check bags for a few days more affordable.

You're on the right track with the advice above.