Sounds like you had some worms. They hide in the blossom, then eat their way out when the fruit develops around them - those rascals. This invites other problems. The stakes are too high for me to suggest anything without at least a photo. I would err on the side of caution.

Google 'Bitter Pit'. Usually caused by a calcium deficiency.

I'd go with bitter pit too and yes they are safe to eat. Last year my apple tree produced apples like this. They didn't store well but I cooked and froze the puree instead.

Yep, bitter pit - use them as quickly as possible, because the brown bits spread if you try to keep them.

We get this is. As said it is caused by a Calcium deficiency. It occurs when trees are grown in acid soil or if there is some problem with the tree taking in Lime. I treat our susceptible trees by drilling deep holes around the feeder roots of the trees and putting Calcium carbonate crystals in. This seems to work. In dwarf trees you can spray the open flowers with a Calcium solution, but never tried this so cannot say if it works or not.

Ensuring regular water supply and mulching annually in spring should help with this, if its not a problem every year - if water is in short supply, the tree can't take up enough calcium for its fruits.

I wouldn't advise you to eat anything without proper identification of the problem as well as secondary problems.

Here is a website which goes into more detail of identification, causes & prevention.
https://www.rhs.org.uk/advice/profile?PID=607