Personally I did do as indicated for the bottom left, but I have no idea how the "painting" of the last 2 columns works! I assume at least three people understood this, as three entrants have been ennobled as correct solvers, but I still don't see it!
I was convinced that this last "line" would finally confirm the unchecked letter (the N in DOBBERS-IN), so I spent hours trying to spot how it would work - and failed! I think it's a bit dodgy to say the least to expect solvers to fill in a letter based only on the fact that there is a word in Chambers, albeit not actually listed as it's pluralised, which fits all the other checked letters in the answer if we add the missing letter. We've been referred to Collins and OED in recent puzzles, do we have to scour all other dictionaries to ensure that DOBBER-SIT, DOBBERS-IT or DOBBER-SIP for example don't exist somewhere? It could have been so easily checked with one more clue!
I think it would help if the "three of your words ... and all the rest" in the painting thing were revealed in the solution. There ARE three of the original words in these columns, but there are also the same in the first two columns, so I am sure I'm missing something.
Also, a minor complaint, the "one straight line" involving going under the table, cannot be "one straight line" as printed in the solution as there are discontinuities top and bottom - it must surely run corner to corner of the columns so that it continues unbroken underneath the table?