I'd have to disagree with the 2nd poster here.
I've seen some pretty bad results when too much extension was used up inside the mast.
This can definitely "stiffen" the bottom of the mast and can make the lower camber or even RAF battens very hard to rotate. (Your situation A).
The reason they make several length extensions is so you can use an extension that supports the mast well, but does not stiffen the bend characteristics apprecialbly.
In your situation B, even a 45 cm (long/tall extension has an additional 15-20 cm of tube at the top to support the base of your mast.
So, you can go out to the full adjustment (45 cm) and feel fairly confident that you are not unduly stressing your mast.
Certain size rigs (7.5-8.0 mostly) use 460 (IMSC 24-26) masts that are extended out to near 500 cm.
The bend characteristics of a 490 (IMCS 28-30) are just too stiff for these sails to work well on a longer mast, so the luff length specification is 460 cm + 35-40 cm of extension.
You could use a 490 mast here, but the sail would not twist of as designed and it might not rig well at all.
Also, if you are a big guy, then maybe (only maybe) you'd like the feel of a longer stiffer mast in this type of sail, but most sailors < 100 Kg. seem to prefer the "best" mast in these cases.
So, your situation A is almost worse than situation B.
I have seen several masts "point load" at the top of a long extension inserted 35 cm or more up into the mast.
The "point load" simply "snaps" the mast right at the top of the extension because the mast cannot bend with the extension inside it down lower and consequently breaks off at the first point were it can bend.
Hope this helps,