It should come as no surprise that a diet that stresses roughly
equal portions of fresh meat and fresh veggies as the primary
food, with smaller amounts of fruits, dairy, eggs and nuts, is an
extremely healthy diet.  Humans are evolutionary omnivores who
are the top predator on the planet so we have not had time to
evolve into an optimal diet so eating with our omnivore digestive
tract in mind will yield good health with very little effort other
than
an eye to variety and freshness.

The level of effort for Atkins or other low carb plans isn't that high
once you get used to it.  What meat is on sale today?  Have
that one as long as you've had other types recently.  What
vegetable is sale today (including roots while on maintenance)?
Have that one as long as you've had other types recently.  Also
have a salad.  Done.  It actually can get that simple if you want
to focus on simple, though it gets pretty dull doing it that simply
for a long time span.

Being a veteran vegetarian you're used to putting in a lot more
effort towards balancing nutrients than is necessary on a low
carb meat-and-veggies based system.  All three macronutrient
types basically take care of themselves just by having some
variety in types of meat and veggies.

Since your diet is low carb and low protein, you must be eating fat.
Instead of worrying about what's happening from eating some nuts while
on a diet that is unsustainable anyway, I'd be focused on a sound,
proven diet strategy that works.  For me, that's Atkins.

It depends on what you mean by safe and sufficient.  You are currently
doing an experiment at near half the normal minimum recommended protein
grams so unless you're paying close attention to the individual amino
acid counts you're now below the safe threshold.  Even with counting the
individual amino acids it's unlikely you could cut your protein intake
any more safely.

If you mean alternating your current experiment with a plan with around
twice as many protein grams, as long as you are currently counting by
individual amino acid type, it should be okay.  Your current system
takes a lot more work than any system that uses meat for protein.

Body builders often alternate between low carb and low fat plans.  It's
effective at pushing their body fat percentage lower than can be done
with simpler means.  Being already underweight I don't see how some sort
of parallel strategy based on alternating protein levels could benefit
you.  Driving your body fat lower is not going to benefit you.

What I do to control protein intake doesn't apply to you.  I do not
sometimes to drive fat out of storage without changing my total
calories.  I trade larger portions of lean meats for smaller portions of
fatty meats for about the same total calories.  That reduces excess
protein calories burnt as fuel in exchange for excess fat calories burnt
as fuel.  It sounds like a net zero if you don't know that leptin pulls
fat from storage to drive basal metabolism.  But even doing that I
bottom out at twice your current protein grams so I don't have any need
to count individual amino acids grams.