The whole SABRE and Skylon concept has been explained and gone over almost to death by some very interesting people here:
The Reaction Engines Skylon Master Thread (1)
The Reaction Engines Skylon Master Thread (2)
The Reaction Engines Skylon Master Thread (3)
It's a huge amount of reading and needs to be summarised but you can hardly imagine a question which hasn't been very well addressed as far as anyone can with a development project and within the understanding of a lay person such as myself.
I think the point about whether the Hydrogen can absorb all the heat is answered by the fact that they don't liquify the incoming air. They just get it very cold so it becomes compressible. This is the key insight which makes their effort different from ones in the past - that a rocket can run on cold, non-liquid gas. The amount of hydrogen needed becomes a great deal less than in a Liquid Air Cycle Engine.
If you read the threads you'll find that their weight margin is very good.
SpaceX is interesting. They may and may not reach their reusability goals. Someone pointed out that a rocket booster containing explosive fuel and kept aloft by a delicate mechanism is not something that you would want to have any failures with if you were one of the people underneath who is waiting for it to land. Also not something you want to have within many many miles of a city in such an event either. I don't really know because I am the farthest thing from being an expert.
There is also an aspect known as cross-range which controls the difference between where a rocket can take off from and where it can land and having a big one like you get with a winged vehicle is an advantage.
Regards,
t43562