Obviously, it's your choice.
I live in CA and when houses change hands there is a specific item on
one of the forms regarding smoke detectors in the house. They have to
be there. I don't know if this is federal or not. If we were talking
electronic door strikes or extra doorbells or something like that, I
can understand messing with it. But on something that is specifically
called out on one of the forms of the transaction. Obviously some
people think it is important.

I've seen other posts that call out, houses built after a certain date
and houses built before a certain date regarding if they have to have
interconnected smokes or not. Or doing additions to an older house and
having to upgrade the smoke detectors from individual to interconnected
in order to meet current law. I am not familiar with those laws at all
but if they do exist, obviously it is pretty important to some people.

Regarding cutting the wires and putting it back. I work for a company
that makes products that customers buy. We get customer returns. It
is obvious when people mess with them. How much do you think we should
stand behind products that have obviously been worked on by a customer?
And our products have nothing at all to do with life safety. I've got
to believe that FirstAlert or Firex or whoever else, have legions of
really good lawyers that have crossed this bridge before. How much
idealism can stand up to that? how much vanity can stand up to that?
You said, 'That puts everything back the way I found it, electrically
at least. Do you think that will leave me safe?" I am sure you are a
great solderer, but I am also sure that there is a pretty good chance
that the original manufacturer can detect modifications. How much is a
new replacement smoke detector? $10, $60 max? Just change it out with
a good one installed. Leave the modified one in a box with
instructions on the kitchen counter if you just have to but let the
buyer re-install it themselves, not you. Another way to look at it,
let's say there is no fire. The buyer doesn't clearly understand the
change you did and, during the transaction, says, 'hey, that's great,
thanks!'. Then, a month later they read what you did, don't want it,
and someone tells them this is a big deal so they overreact and they
have a lawyer write you a letter asking you to put a good one in or
else. What are you going to do? why hassle with it. Just buy a new
one and sell the house with the new good one installed and your
modified one in a box on the counter with instructions.

Another poster on this thread mentioned not messing with the originally
installed interconnected smokes at all but just dinking around on a
separate set of interconnected smokes. I think that is a great idea.
the best idea. Have your sandbox to play in, but have the one that
counts in perfect condition, unaffected by the sandbox. Build your own
sandbox exactly how you want to, play to your heart's content.
install, experiment, test, debug, have fun. But leave the original one
intact.

You mention you are 58 and have been doing this for 50 years. Surely
you can afford two or three or a dozen smokes from Home Depot to play
with. Obviously you are smart enough to design your own
interconnected system out of those smokes. So do it that way and be
safe.

Hopefully I didn't step on any toes.
Have a good holday!