Cracks in various 90-94 Viking and Post Questions

It all depends. Most all that I've seen aren't deep enough to allow moisture in. But I did see a couple that would be suspect.

Water ingress is a possibility as @Capt J may or may not agree with.
They can be fixed.
DIY is possible.
Shipwrights are recommended if you want professional experience with proper facilities, tools
for the job in which case you are in for serious labor hour dollars.
It depends on your goals.
You could survey the areas for water intrusion with tapping and or moisture testing.
Having said all that they are rarely a deal breaker, but should be taken into account.
I don’t like them unattended, but bazillions of boats float around quite happily with them.

You're dealing with 30 y.o. boat, and there was a lot of learning and experimenting going on around that time. So not unusual, especially in stress points like hinges and corners. Are we talking about cracking in the gelcoat, in the fiberglass under the gelcoat, through them both or just crazing? It's all fixable and a lot of it DIY depending on your skill set and desire, For small repairs where an exact color match isn't critical you could even use Quick Fix. Good stuff to keep a tube of around. Test for retained moisture and soft spots. Those repairs will get deep and expensive. Closer to the surface it's just part of maintaining a boat.

Not necessarily. Older Carvers are known for rot around stanchions near the boarding areas where water collect. Someone posted several years ago about cutting out and replacing a pretty good sized area several years ago.

#3 looks the most serious. The other two NBD. But put a moisture meter to them and push on the deck to see if there's any delam or sponginess. If not that' a DIY project, again depending on your skill set and desire.

If you walk on the affected area, how does it feel ?

Just get a $40 Ryobi moisture meter at Home Depot. They work well for things like that and easy to use. I've never heard of water intrusion issues on the vikings or posts that had this issue in those years, but it's not impossible.

NYCAP- this was a bad brand of gel coat that was used by Post/Viking and a few others and around late 90's to early 2000s.

Even earlier. I remember. My first fiberglass boat was a 1950 which we got in 1956. Lots of experimentation since then trying to build strength and lessen weight and amount of material. I don't think this was involved with that batch you refer to but same thing. There was a lot of trial and error during the late 80's / early 90's and beyond.
That the areas feel solid probably means an easy fix but as J says put a moisture meter on it anyway just to be sure (and go over the hull while you're at it). Do your best to keep weather off it till you make the repair. In fact I'd say open the cracks up a little a do the repair with Quick Fix. Maybe that'll be all you need, but it won't make a professional repair any harder and it'll keep the moisture out till you get to it.

And the OP said he's looking at '90 to '94. Long before that.

Stress cracks in the gel coat are pretty common across brands and eras. These look like typical gelcoat cracks to me especially if the area feels solid

the hardest part is to match the gelcoat. Some guys are really good at it, others not so much. Sometimes a repair looks good but then a couple of years later it ages badly.

There you go. And like Pascal pointed out even if you get a good repair it will probably become evident after it ages a couple years. That's why most boats are less concerned with the gelcoat matching and paint over. So if the fiberglass underneath is good and it's not delaminating do the cheapest but smoothest repair you can with the closest color match you can and plan on a paint job. The funniest thing I find is that one of the biggest things that sold people on fiberglass over wood was that it would end all that painting, but soon enough fiberglass boats were being painted and requiring more expensive and finicky paints.

Nobody is going to repaint a boat because of a few gelcoat cracks here and there. You get a good guy to repair it and it will last. I ve had a few chips and things repaired on the boat i run 4 years ago and I can’t tell where they were. My guy took a small door to a place on FTL who perfectly matched the gelcoat.

If anything it s probably easier that blending in Awlgrip or Imron.

You just need a qualified guy. Or Gal... or Undetermined.