There's also the insurance issue. I don't think you can expect
an instructor to get in a privately owned airplane unless they
are "named insured" on the owner's insurance policy and the
policy provides a level of coverage that the instructor is
comfortable with. (If I was the instructor, I would also want
to know the aircraft's maintenance history, etc.)

As for the 6gal/over gross, I found that comment confusing.
(Unless either this individual or all the instructors they've
talked to are big, heavy types, I would think that the W&B
would be ok with 6gal in the tanks, which would be ok for
a 45min spin trip, I'd think?)

Doing intentional spins when outside the utility W&B limits
is definitely a no no. (The W&B for typical 150s around here
would allow student+instructor+6gal, unless the student and/or
instructor are pretty darn heavy.)

And I'll add my name to the "don't try the first spins on your
own" list, rick