> Can an equivalent length of available-size blades be welded together (maybe trimming the ends first, if they lack teeth)?
I don't think so, these blades are very narrow and thin -- 0.2mm x 0.3mm cross section at the spine -- and I'm pretty sure a weld would not hold and if it did, you'd still have to grind it down flush. And I'd guess the joint would also fatigue which is an issue since the blade needs to get wound/unwound repeatedly during cutting. Also, I think the heat of the weld would probably effect the already hardened steel.
> I wonder what the required blade tension would be
The tension on existing scroll saws is 10 - 20 lbs for blades of this size.
> how machine will maintain tension on the blade
The blade gets unwound off a spool which maintains the tension through a torque motor.
> I wonder why the blade needs to be so long.
Minimum usable length is ~10m, but I figure if you can make 10, you can make 50. The longer the blade, the longer you can cut before you have to rewind. And the longer the blade lasts before the teeth are dull and needing to be replaced