Medium Density Fiberboard (Common: 1/4 in. x 2 ft. x 4 ft.; Actual: 0.216 in. x 23.75 in. x 47.75 in.)

This will probably work if you only want a decorative shape to put on your wall. It is only less than 1/4" thick (about the thickness of corrugated cardboard), so it would not be suitable for any load bearing. I used it to make glue-on shaker look stiles and rails for cabinet doors, glued to flat faced doors. Turned out nice for this purpose.

Each panel weighs about 9 lbs.

I would not recommend it as an underlayment. Any moisture would cause it to swell

No

The hamster might eat through it!

If it is sealed first on the end grain. I would suggest spray shellac to seal the ends. Remember a lot of cabinets are made from this so as long as it is well sealed you will be fine

That is a very open-ended question, like asking how much weight a tree limb can bear and how much deflection are you willing to accept.. If you were thinking of using this as a shelf, or shelving, I would not recommend it unless it was fully support on all four sides and in the middle, as a minimum.

This material would be a bit thin for a 4-foot ramp. Plus thr surface might prove to be too slippery. I would go for something thicker with a rougher surface, just to be on the safe side.

You should be able to get two 23.75" half circles out of one piece of this fiberboard. When joined together they would make a 47.5" diameter table top.