I asked several employees on the Navigator about the Crew Welfare fund, and they said they have never received any benefit from the fund. Some did not know it existed. It is supposed to be used if someone's contract is not over and they have an emergency and need to go home. Some long time crew told me this. Officers said otherwise - that the fund is used for things like crew parties and the emergency as noted above. Letters to the Captain are special and highly regarded and put on the crew bulletin board.

I personally have chosen not to contribute to the fund on Navigator. We did have a waitress that went over and above to make our cruise special; We had a party of 8 nightly in her area and we were going to collect for a tip and then decided that we would not as a group because of tipping included on RSSC. I gave her a token amount because I thought she made the trip extra special - Just a $20. as a minor thank you. I rarely have tipped on RSSC because of the policy.
On the Mariner, it was as you were told - they did get personal DVD players.

The spa employees are not paid as the regular staff. They actually get commission as do spa employees on land, so a tip is appropriate there.

I provide payroll services so this is of interest to me as many of my client businesses are spa and beauty and other tipped employees.