Answer to all questions posed: It depends on alot of things.
#1: I think it would be nice to have lines' weight marked by the factory. Cortland prints it, Teeny uses different color running lines, I mark them with a sharpie pen (long mark = 5 or 10, short = 1). I always forget what color from what brand is what, so colors make the least sense.
As far as marking the overhang, a bump isn't so great because everyone and every rod likes a different amount of head out of the tip. Some people like to tie in a nail knot of mono or use a sharpie to mark their sweet spot. A color change for this works well because you can see how much you like to have out.
2# : If it casts well, resists tangles, and is durable.
3# : I have only used interchangeable heads on my spey rods and fishing for steelhead. I hate the clunking feel and winding up 30' of shooting head is a pain. I would rather fish full lines on spools, but the potential to save money could be a good selling point.
4# : It depends on which rod I would be using it on, and how long the head was. A Sage RPLXi and Winston XTR have different needs. It would be cool if they offered saltwater lines in 50 gr. increments, and actually weighed the lines and marked the box (eg. : 329 gr.). When I weigh the lines, I am almost all ways surprised.
5 # : Both 26 and 30.
Until then, I will buy different lines for different rods, mark and chop and splice.
The Rio lines are very good, and I like the Airflow fast sinking heads. I find that I am using the intermediate lines less and less, and useing a floater more often.
Juro, if you need a line tester...