You're in Hong Kong, right? I'm not familiar with that model, but if
it's like the U.S.-market 1993 LE (black with red interior and BBS
wheels) it has Bilstein shocks, which are a bit harsh.

No, it's not a good idea to delate the tires to soften the ride. 28-30
psi works well for most Miata owners; don't go softer than 26 psi. Some
tires ride much better than others--the Toyo T-1S is one of the best, as
well as one of the best performing. But lowered Miatas tend to hit the
bumpstops a lot, which produces a jarring ride, and the only solutions
are raising the ride height or fitting stiffer springs.

Not normal. Perhaps the rear brake calipers need to be adjusted. Look in
the miata.net Garage section for instructions.

Replacing the rear curtain will not cause leaks; it's not a particularly
challenging task, and you could do it yourself. However, for $500 you
could replace the entire top. The cheap solution, if your zipper is
still in good condition, is to visit an auto top or boat upholstery shop
and have a new piece of plastic sewn into the top, without removing the
top from the car.

The parking brake need to be adjusted. The adjustment is under a bolt on
each rear wheel hub behind the brake caliper.. www.miata.net  has a garage
section that should describe the needed operation.

You will also find niformation detailing various top and rear window
replacements and alternatives.
The  reference to leaking is undoubtedly related to a plastic part that
often cracks when removed top replace a top or rear window. Eithe the
plastic part is replaced when it cracks, or it can possibly be sealed with a
non acedic form of RTV. The part referred to is not visible until the nuts
around the bottom rear of the top are removed, and the metal top retainer is
also removed, allowing the bottom of the top to be loose, and not bolted to
the car body. Naturally, all of this is under the rear deck carpet, so the
first step is to remove the cross brace behind the seats, then the carpet.
You will likely need a tool to remove all the plastic buttons holding the
carpet in place. On an older Miata, you can expect several of them to
shatter, requiring replacement. Fortunately, the usual black ones may be
available at a parts store. The other colors are likely to be a dealer order
item.  Trussville mazda  is a discount dealer with web presence. Again, I'd
refer you to www.miata.net for a list of various vendors.