Yes, you have a right to keep the cat you love safe. It's your choice
whether
you want to. Is she good at hunting cars and taking down sadistic thugs who
think it is fun to hurt little creatures?

odd to hear of a vet who does not like chipping but approves of
collars, usually the other way round

I know that this a difficult subject & that everyone deserves to do the right
thing by their cat .
Every single one of us has a different opinion to others, according to where
we live, the predator threat, cars & even other human beings too.

I used to feel exactly the same way that you describe in your post. Ringo was
a big fat orange tabby who chose us just as  Marlo seems to have chosen you.
I felt guilty about the fact that I was taking his liberty away. I found out
afterwards that his owner died, & that his daughter had tried on several
occasions to keep him @ her house in the same village, but he kept escaping &
coming back to his old neighborhood. For several weeks he had visited our
community feeding bowls, so we knew of him, but not very much.

One night there was a knock on the door & I went to answer it, & he toddled
straight past me into the kitchen where he sat @ the food bowls until he had
his fill. I was left standing @ the door with my jaw hung open!

Ringo was with us for a few months & we allowed him the freedom he wanted,
although the rest of our cats were indoor cats. I thought it simply unfair of
me take that liberty away. One terrible evening, we suddenly realised that he
wasn't around which was quite unusual, so we gave him an hour or so, then
decided that we had better start looking for him, because he was normally
always home before dark.
Eventually, my son who had bonded with him, found him almost under a hedge in
the meadow behind our home. He had been shot with a BB Gun @ almost point
blank range. It was obvious that he was trying to make his way home, but
never made it that far.

We took him to the vet, who told us to prepare ourselves, because he thought
we would loose him. In fact, he got through the operation & did recover;
Actually he is still recovering. now Ringo is around 8ish years old, so we
think he was very lucky.
The reason I am telling you this story, is not to make you feel guilty, or to
try & persuade you to do one or the other. It is merely to share the fact
that I felt exactly the same as you do right now. It never once occurred to
me that Humans would be the predator that brought him down, if you see what I
mean? You must do what you feel is right for Marlo. It could be that you live
in the middle of a fenced off compound that she could never get out of for
all I know!

My point is that you never know what will happen, or to whom or why, but you
are assured by containing them, that the chances are far less likely. I would
never chastise anyone for what they feel is right for their cat. In the end,
we found that the best answer was to make a Big cat pen, so that the other
cats could have a bit of liberty, & Ringo's was not withdrawn.
You will know what is right for you.
Best Wishes with this decision. It is not an easy one & I don't envy you.
Good Luck,
Sheelagh >"o"<

Many former street cats make wonderful pets; a steady supply of food,
clean water, affection, and shelter is hard to resist.

But, having been more wild, she is more comfortable with the outdoors
than many cats are, and has no fear of wandering away from familiar
ground.

But keeping her in your yard will ensure a much longer, healthier life
for her.