I'll preface my reply by saying I'm not a mechanic or engineer, nor am I a particularly skilled home-mechanic.
1. This would sound right to me, based on my understanding of how hydraulics work (the liquid can't be compressed beyond a certain point, which is what provides the stopping power). The overall volume and compressibility of the fluid in the tube determines the lever throw, not the cable length as in rim or mechanical disc brakes. As such, it wouldn't be adjustable in hydraulic discs, beyond shortening or lengthening the tube. I have noticed that the lever throw on my rear brake is a bit longer than on my front brake. My assumption is that's because the rear brake line tube is longer. Someone more qualified may be able to correct me here.
Your 1st question is also somewhat linked to your 4th. I haven't had issues with the length of the lever throw. I've found that it's actually really good, shorter and firmer than on cable brakes, except when there's air in the lines and the brakes need bleeding. In short, if the brake lever throw is too long, bleed the lines and make sure you then fully top up the fluid.
4. I definitely don't bleed my brakes every 6 months. In the 4.5 years I've had the bike, I reckon I've done it 3, maybe 4 times, so probably around 12 months is more likely. I notice it's time to bleed the lines when the brake feel becomes mushy and the lever throw gets long. Like I said above, normally it's quite firm and short. The bleed frequency may depend a bit on whether the brakes use mineral oil (like mine) or DOT brake fluid (the 2 are not compatible and using the wrong one will wreck your lines). I did some reading before I bled my brakes the first time, to make sure I got it right. From memory, DOT fluid absorbs air over time, whereas mineral oil does not. With mineral oil, the air presents as separate bubbles within the lines, rather than getting absorbed into the fluid. Again, someone more qualified may be able to correct me.
2 & 3. I don't know about weight differences. Mine is a steel frame commuter, with full mudguards, pannier, etc, so weight wasn't and isn't a big consideration for me. I will add that my current bike has a carbon fork, as opposed to a steel fork on my previous bike, my guess is that's to compensate for the overall weight increase of the disc brakes.
On the hydraulic V mechanical discs discussion, I only read some of the comments, I didn't read them all. My view is that hydraulics are manifestly better, both in terms of stopping power and ease of maintenance. If you're going to the extra expense and weight of discs, I wouldn't bother with mechanical. Unless of course you have a very specific requirement, such as remote trekking where availability of parts is limited and critical. That doesn't sound like what you're going for though. For everyday, all weather use, I reckon hydraulic discs are clearly superior.