"A welded joint of any description in Al is never close to the parent material in strength. You know this. Therefore, in comparison to a single
extrusion, a welded spar is compromised."
Do not forget to tell this to all the people that weld on mast since decades, they probably did not realize that.
"dpt isn't going to tell the OP whether the joint has complete penetration & fusion & whether, or not, there are below surface gas pockets. fair enough?"
Sure, that is what I said - any open to the surface defect, this include any LOF that are open to the surface of course...but not visibles by bare eyes in visual inspection.
Now in term of defects, what will your criterias about LOP if you found some? How long ? How deep? based on which code or standard ?
let's go back on earth for a while, we are talking about leisure boat, 32', a good visual and an easy DPT will be way enough on the weld to make sure it is OK.
And you won't see anything form the inside because of the sleeve.