Olympia 24 in. x 24 in. x 38 in. Composite Vinyl Cupola with Copper Roof

Yes...there is a stop built into the inside for the rod to rest upon and there is a hole and sleeve in the top of the copper. If you do not use a weather vane they give you a plug for the hole.

I think so it has a large hole at the top

The Olympia cupola measures 24 inches square at the bottom of the flared base and at the widest point of the copper roof.

On top of the shingles is the most common method. No problem. If you have a ridge vent you will have to eliminate the vent in the area of the cupola and either shingle it or do a flashing and seal job before installing the cupola to prevent leaks

No. This cupola only accommodates roof pitches from 2/12 up to 10/12. Some cupola manufacturers offer a taller base to allow for the deeper cutout required by the steeper pitches. I recommend calling them to inquire about it.

Yes you can

Yes

The typical weather vane for this size cupola is approximately 24" tall with a 3/4" diameter rod to attach through the hole provided in the top of the cupola roof.

Good up to 10/12 pitch

That size may be inadequate to ventilate the attic space of a 24'x49' building. For a 49' ridge line you're probably better off with two cupolas for both function and aesthetic reasons. For unbroken ridge line the general rule is 1.25" of cupola per linear foot of ridge so in your case 49 x 1.25 = 61.25" x 61.25". That would mean a single 5'x5' cupola in the center of the ridge. Two 48"x48" cupolas would look much better. For purely ventilation purposes and disregarding the look factor you would need two of the 24"x24" cupolas to vent 1176 square feet of attic space.