This vaguely reminds me of Disney's "Where's My Water
" game. Sure there are obvious differences, but I think it's not hard to see where I notice the similarities. The idea could
work but the mechanics definitely need to expand a bit for a complete game; you can try the Disney game (or better yet, just search Youtube for solution walkthroughs if you don't mind solution spoilers) to get a sense of how they gradually build up the mechanics starting from the basics, for inspirations on how you may apply to your game here.
Like namida says, use left-click for as many things as you can without causing mix-ups; right-click should pretty much only be used for scrolling based on what you have so far. I'd also add that it may be useful to show a software mouse cursor within the level area itself, one that changes graphics for example based on which tool is currently selected (so user doesn't have to keep looking at the bottom and away from where the action's at to see which tool they are currently using). Audio cues on reaching full/empty bucket will be very helpful, since again the user's eyes will likely not be looking at the bottom while using tools.
I think it's fine to use gnomes and not have them do anything. I mean, sure if you market the game as related to Lemmings, inevitably some people will expect the gnomes to be able to do things on their own and perhaps get disappointed that way. Avoiding the lemmings association might actually help a little in this regard I think. There are plenty of physics games where the main character(s) doesn't really participate much in the level action except for basically being the goal (eg. Cut the Rope). That being said, even without directly assignable tasks to the gnomes, it is probably worth exploring some ideas of having objects in the game that gnomes can interact with (beyond the deadly lava), for example switches activated by a gnome walking past, to turn other things on/off.
I do have to wonder whether the core mechanics of, basically, shuffling sand around may get a little tedious at times. In comparing with Where's My Water for example, IIRC there the main moves are basically turning spouts on or off (so a fairly lightweight, binary action), and removing terrain by swiping your finger around to create a path for the water. The latter is interaction-heavy but helps that there, it was designed primarily for touch so it feels intuitive and even somewhat fun; I imagine that even just having to use the mouse to do the same would probably already feel less fluid. In that sense I felt that the sand-shuffling in your game can potentially get slightly tedious, even though I realize it's not completely avoidable as a core mechanic. It might help if earlier levels at least feature much larger bucket capacities so that user is not forced to switch between sucking and dumping sand so much. Or maybe add a keyboard shortcut to make it more lightweight to switch between the two tools.
Anyway, I completed the level. It was not bad, I think I enjoyed it overall. Looking forward to see how you expand upon these initial ideas into a more complete game.