ideal spreader attachment

[Brion Toss]

Let's see, first a rigid connection is not necessarily a bad thing, assuming that the lead of the shroud is fair over the tip of the spreader; you don't want that shroud to be pulling that spreader tip significantly fore or aft. This can be an issue with bendy masts, but even here the spreader angle can be set for the fairest leads when the loads are highest.
Many spreaders that have "some allowance for the spreader to move" are just originally-rigid setups that have become deformed from unfair loading. There are spreaders intentionally set up to swing, but these have their own issues, especially the tendency of the spreaders to slam around on the leeward side, imposing very high fatigue loads on the components, including the mast. It can be done well, but it must be done carefully, and for most boats it just isn't worth the effort.
Welding on aluminum spars is not straightforward. It is a tricky job, requiring a high degree of skill and knowledge. Many, many rig failures are directly attributable to weld failure. Re the assembly in the picture, I have no way, from here, of knowing if the right stick was used, that penetration was complete, that surfaces didn't deform, etc. The thickness of the weld looks quite, um generous. In general I prefer no welds on a mast, partly because it is so difficult to find someone who does it well, and partly because there are usually better approaches.
For instance, the through-bar, with spreaders socketed over, its ends, is strong and simple.
Compression from the spreaders is usually not an issue, given the relatively low magnitude relative to bearing surface.
Fair leads,
Brion Toss