We'd like to view the Leshan Buddha both from the stairs and from the boat.

1. Yes, it is possible to do them both. As I know, the hiking and boating have the separate start points. If you want to experience them both, then you may need to afford for them respectively. Since the boating may only have around 15 minutes at a cost of CNY 70, you are advised to hike to enjoy the grand Buddha. In this way, you can have a close-distance view. As for the time, it varies from the different physical conditions. Generally, it may take at least half a day to do the hiking part.
2. There are lots of high speed trains departing from Chengdu East Railway Station in one day. You can buy the ticket when you reach the railway station. Have a nice trip.

Visiting the buddha in both ways are doable, but it is suggested to choose either of them. You will look at almost the same thing. Why choose both ways? Personally, boating can be a good idea. You can see the whole view of the buddha and take some pictures onboard. A single journey takes no more than 20min.