I think there is a fighting chance that you are remembering Operation Chaos, by Poul Anderson. A little Googling tells me it's had several different cover illustrations over the years, but here's a scan of the same artwork that's on the front of my old copy.
Originally Anderson wrote some fantasy stories, with the same co-stars, and all set in the same "alternate timeline" where magic was very well-developed in the Twentieth Century, with those stories being published separately in magazines. Then he combined them, did a little rewriting, and ended up with what was first published in book form in 1971 as Operation Chaos. Many years later there was a sequel, deliberately written as a novel instead of being a "fixer-upper" from previous shorter works: Operation Luna.
The narrator of these stories is Steven Matuchek, a hereditary werewolf. In the first story ("Operation Afreet"), he is introduced to a charming young lady named Virginia Greylock, an "adept" (i.e. an expert witch). They work very well together in a dangerous commando-style mission, and later on they become a happily married couple.
Here are some passages from the first part of the book -- what was originally published as "Operation Afreet." I'll try to pair them up with relevant bits of what you remembered and described.
I think there were werewolves, and probably were-bears or were-something-else people too, some explanation about conservation of mass during transformation, maybe also a difference between transformation by using a magic spell and transformation from inborn ability.
Steven Matuchek says:
I weigh a hundred and eighty pounds, and the conservation of mass
holds good like any other law of nature, so I was a pretty big wolf.
And later he says:
You see, while practically anyone can learn a few simple cantrips, to
operate a presensitized broomstick or vacuum cleaner or turret lathe
or whatever, only a small minority of the human race can qualify as
adepts. Besides years of study and practice, that takes inborn talent.
It's kind of like therianthropy: if you're one of the rare persons
with chromosomes for that, you can change into your characteristic
animal almost by instinct; otherwise you need a transformation
performed on you by powerful outside forces.
I think there might have been a part where a wound while transformed stays after the end of the transformation - the woman werewolf was injured?
At one point, Matuchek tells us something about the general rules of being wounded while in your magical were form:
A natural therianthrope in his beast shape isn't quite as invulnerable
as most people believe. Aside from things like silver -- biochemical
poisons to a metabolism in that semifluid state -- damage which stops a
vital organ will stop life; amputations are permanent unless a surgeon
is near to sew the part back on before its cells die; and so on and so
on, no pun intended. We are a hardy sort, however. I'd taken a blow
that probably broke my neck. The spinal cord not being totally
severed, the damage had healed at standard therio speed.
I'm not sure, offhand, that the female star ever turns into a wolf, but I did remember that, at the end of "Operation Afreet," having had a big piece of his tail bitten off (when in wolf form) by an enemy weretiger, Matuchek described the aftermath of that injury (relevant to what he'd previously said about "amputations"):
I don't have many souvenirs of the war. It was an ugly time and best
forgotten. But one keepsake will always be with me, in spite of the
plastic surgeons' best efforts. As a wolf, I've got a stumpy tail, and
as a man I don't like to sit down in wet weather.
That's a hell of a thing to receive a Purple Heart for.
I think there was some sort of explanation about having to say seemingly irrelevant words for casting spells, maybe they were chosen by the person initially creating the spell.
Steven Matuchek explained to us that it was necessary to use an exotic language to cast spells. Although, from various references throughout this book, I got the impression that the most important thing was: "Is this an exotic and unusual language by your standards?" (Though it wouldn't have been anything special by the standards of people of a past era who used to speak that language all day, every day -- such as Latin in the heyday of the Roman Empire.) At any rate, here's a sample of what he says (not going into that much detail here):
I went over the spells I'd haywired. A real technician has to know at
least one arcane language -- Latin, Greek, classical Arabic, Sanskrit,
Old Norse, or the like -- for the standard reasons of sympathetic
science. Paranatural phenomena are not strongly influenced by ordinary
speech. But except for the usual tag-ends of incantations, the minimum
to operate the gadgets of daily life, I was no scholar.