EDIT: This answer is a bit old. Now we know much more about PoS and validation details, but don't hesitate to ask new questions on a new thread.
Also, meanwhile Golem Network appeared and is a pretty way to recycle GPUs power (see the end of my answer where I express fear about GPU recycling). GPUs would then be used for what they were built for: rendering and massive calculation. Many other projects may appear and I'm happy that my prediction about people being creative were true ;)
TL,DR: I may look pessimistic about POS but there is a huge chance that people creativity makes something good of it. However there will be a huge amount of GPU to recycle, so we have to find a solution now.
Now the long talk.
As I can see it, POS transition was primarily started to lower the ethereum network power consumption and be more ecological. But the drawback is that tons of GPU will be left useless (even if there is still some POW coins out there, but come on, mining LiteCoin to convert it to ether?) and trashed if not sold. I really mean tons. Really ecological? One may do the math but I don't have the figures to put in the formula. Also POW mining looks to be a more accessible, decentralized and incentive way to be part of a network you care about. Will you be really a part of something ruled by a bunch of big firms that hold ( or hodl) a lot of ether? I think this is the opposite of what ethereum was build for first. But, answering your questions:
1- as I understood it, what we call miners today will be validators. They will exist in limited number as POS is described for now but this may change. So not every people having a node will be rewarded. Only a few big corporations with a lot of stake to participate in POS will be able to be rewarded. The more you have, the more you gain... All our rigs will be useless even for being recycled into POS servers as the need is really different and don't expect to build a high availability powerful server on an Asus btc motherboard. So except if you want to mine some other coins or sell your rig, this is good to trash. (Note that selling rigs and GPU will be nearly impossible when POS will be there due to the gigantic gpu offering opposed to the tiny demand as none will want POW hardware anymore)
2- new miners will have some way to get ether. First one is to buy some at an exchange using fiat currency. This is how most of non miners do today and this is how I stared before being a mining addict :D.
They will also be able to change other coins like BTC or LTC they already have into Ether using an exchange or the now famous and beautiful ShapeShift (I love you guys even if all this hack thing is not completely clear to me... But this is another thread subject).
Note that if a miner don't want to use fiat currency at all, he can mine LTC for instance and then change it to ether. But this is not really efficient, I admit.
There's some other way though. Some can sell goods or services for ether. I already saw a young designer offering logo creation on Reddit for ether.
Gamblers will also be able to win some ether by playing online games like poker. Even if they'll have to put some at first, they will be able to win or loose a lot.
Finally (but not so finally as I may have not thought about anything), you could be rewarded in ether by watching ads or solving captchas for hackers, this is called faucets and I hope it will be eradicated.
3- mining, or may I write validating, will be the only way to create ether from void. So in order to keep the network ether creation rate, POS will have to be attractive. This will be done by giving a few people the ability to invest ether at a really good reward rate (most classical investment in old world won't be able to compete with POS rates). The more they stake, the more they get. As staking ether will lock it for some times, it will probably be people who owns a lot or people who don't want to use it (Holders). But people who have just some ether and use it won't be part of it even if the network works because of them and they create the ether value by using it and using Dapps and contracts. But they are de facto excluded form the reward.
So yes, you will be able to stake and hold and use it if you have enough ether.
4- as I understood it, validators with their huge servers will process all. Full nodes that some people will run will only validate blocks integrity but won't run any EVM code or create new blocks. This is what happens today with full nodes in POW. Only mining nodes run the network. I don't know how light nodes will play in this game. They may only have a read and write access but no integrity validating power at all. Read the light protocol paper for more details.
5- computing power will still be redeemable but not as it is today. GPU power will be almost zero value. But computing power on super servers or on big arrays, grids clusters will be very valuable.
Now the optimistic point:
By that I mean that what would be a more available computer than some standard computers joining together to make a big one always on? This may be the future POS pools. People will join their CPU power using some networked clustering tool and each one will stake some ether in a common wallet contract or some forms of DAO. They will then apply to become a validator as one could be and they will be represented by the contract. The contract will receive rewards and distribute it proportionally to the computing power delivered and by the staked amount of each people as it's done in pools with hashrate.
This is the best scenario I see to avoid power centralisation of the ethereum network.
And the pessimistic message for the end.
But all our GPUs will go to trash anyway.