Trish:
You've got a number of things worthy of comment.

1. Without knowing the specifics of your spinal damage, let me offer a broad
comment which may prove totally inappropriate for your case - colored by
having successfully coached individuals with considerable variety of back,
hip and other challenges. While this may not be at all true in your case,
most people think of "I have a back problem" of some sort or other, assuming
it is totally spinal in nature. Their doctors don't help in that matter! The
whole trunk (called 'core' these days as if it's some newly discovered body
part for personal trainers to make undeserved fortunes about) is a complex
of muscles, connective tissue, viscera, and spine. For our purposes, the
whole interconnected unit made up of spine and muscle is the focus: spinal
damages are permanent, while atrophied or poorly conditioned muscles that
support, move and serve as a shock absorber for the spine can be developed -
in some cases, very slowly or with methods promoting rapid muscle growth
(Matrix training). 

2. Your friend's concern about paleo with limited exercise, while well
intended, is counterfactual, rooted in the propaganda of that cartel of
monopolies inclusive of pharmaceuticals, feedlot meat producers, grains,
medicine and insurance, with their mind control police function invested in
the FDA and USDA - in other words, social engineering by means of
manipulative propaganda creates a normative reality or 'common sense' as
equivalencies to post-hypnotic suggestion informing the normal waking
trance.

3. If your type II reversed including no need for meds, there definitely was
a dietary and metabolic component at play prior to paleo, one in which
iatrogenic (illness inducing) medicine catalyzed. Otherwise paleo would not
have straightened you out, especially with limited activity.

4. With your condition, HIT type training should be avoided. HIT was
developed by Arthur Jones 40 years ago, at a time when exercise physiology
barely existed. HIT remains fashionable among men of a certain age - aging
and old, guys who believed Jones' autodidactic, often crackpot, ideas. In
any event, HIT has not withstood the test of time. HIT excludes transverse
movements (rotation), muscles you definitely need to condition to strengthen
and protect your spine/hips. Secondly, HIT was used to sell Nautilus
machines - machines isolating a few muscles by stabilizing all the rest,
thereby artificially relatively strengthening a few while equally
artificially weakening the rest. Our bodies include 656 muscles, while the
average Nautilus machine or most any machine works upwards of half a dozen -
you wonder why rotator cuff surgery has become so prevalent? Machines. Knee
problems - running.

5. If I were coaching you, the stationary bike would definitely be a
worthwhile experiment. Beyond that, cookie cutter answers generally pertain
to personal trainers. Genuine coaching is client-centered, honoring and
respecting the uniqueness of every person. While I love training athletes
and seeing them win - at 65 I have not grown up nor do I wish to! - while
for me the greatest wins come about working with persons like yourself. I go
into such work fully convinced I'll coach you working and winning - winning
your freedom back. To do that you've gotta win every rep of every set,
kicking ass every second of the workout. Also, my backgrounds and
certifications in transformational psychologies comes into play - with me
you don't use muscle to lift weight - instead, you use imagination/mind to
voluntarily gain muscle control for flexing weights. That's an immense
difference because you're activating discrete portions of the brain
connecting to motor units.