The starter motor draws a lot of current (amps). If not for the starter, batteries could be tiny little things. We're talking a good 150 amps or better turning the engine over, and it draws more initially getting 'the ball rolling'. If I recall correctly, the starter motor can draw very much more current in the 'locked rotor' mode. It's like shorting the battery terminals for a couple seconds.
Some people hold the button down for quite a while until the engine starts - car or bike. When the starter gets warm (very quickly) it draws more and more current, so it's best doing several short bursts than one long one. It's much easier on the battery and the starter.
I've dealt with Valeos when they dropped magnets and never had the battery drained. I'm guessing the starter button was still pushed long after the initial click and nothing happening. Just guessing, but I don't think I ever engage the starter longer than half a second.
Good for you rebuilding it. Those things are like a Chinese puzzle - kind of fun, actually. But when it comes to the back area - the black plastic brush holder, be really careful. The springs are very strong and the plastic is brittle - breaks easily. Don't pry on it, or let the spring slip and crash back down or you'll be buying a new one. Thankfully all the parts are available. If you don't glue the magnet back in, new upgraded housings with magnets are available.