(I haven't forgotten about your other questions...but I guess I'm responding to most recent first, at the moment...)
My experience with 2e learners with ADHD is that timed tests can go either way--with the corollary that extended time accommodations can also have results that go either way. Some GT/ADHD persons do better with a little time pressure than otherwise, as it helps them to sustain focus. This tends to be the case more with shorter time frames, or with longer times subdivided into short bursts. Others function exceptionally well on timed (especially rote or overlearned) tasks because of their impulsivity/disinhibition, which reduces overthinking. Again, short tasks are more likely to benefit.
Those who benefit from extended time often do so because they need the additional time to pull themselves back from inattention or distracted moments, with some falling in the slow cognitive tempo variant putatively associated with ADHD-primarily inattentive. Even when extended time is warranted, best practice is still generally to restrict it to +50% (and optimally, only +25%), as there is a point of not only diminishing returns, but actually decreased performance with excessive extended time.
On your second question, it is probably more typical than not for a high-cognitive learner with average executive functions to experience frustration and disappointment (or perfectionism) regarding their own EF skills, because even when function is normatively typical, the gap between one's highest islands of development and one's lowest can feed subjective perceptions of disability. Asynchrony notoriously creates an illusion of deficits in one's personal relative weaknesses.
And, of course, if a learner is receiving academic instruction at their appropriately-advanced academic level, the classroom environment may be placing the EF demands of a chronologically older peer on them, which means their age-appropriate EF skills are being taxed by age-inappropriate expectations. Long-time readers on this forum may remember that I have described my childhood experiences of grade acceleration as following my mother's rule of thumb, which was to place children into grades where we were about 1.5 grade-levels below our true instructional levels, to leave some cognitive energies for managing the increased EF and social reasoning demands of the destination grade. (As homeschool parents, we have accommodated this by adjusting the EF scaffolding, rather than reducing the instructional level.)
