Question on using endomorphism on secp256k1 and negative results
Do you mean Algorithm 3.77 on p. 129?

The algorithm has an explicit case for whether $k_{j,i} &gt; 0$ or not, in the loop in step 8.2.  Obviously a conditional like this is bad news for fast constant-time arithmetic if taken naively.  But as you noted, you can simply do a conditional subtraction in the coordinate field to negate the point, and then do the point addition unconditionally.