To be honest, and not meaning to sound mean at all, but this is all user education. Just sit down and learn how to use your equipment. Spend some time and play with different software until you find what you want. Nothing is automatic. This is not particularly advanced stuff you are looking for here.

Most MP3 players start and stop with the space bar. You may have to configure that quick key, but no matter what, this is computer basic basics here. Is it a huge deal to click a start and stop button with a mouse? At some point a human has to tell a computer what to do. Even if you go iPod Nano, you will be starting and stopping with some kind of human interfacing.

And also, "old" has nothing to do with it. I have customers in their mid 70s who have 3-4 computers networked in their house. One is 81 and he is always looking to learn more.

One tip I would offer is to normalize your songs. You said they came "from all over" and I will take a stab that some are recorded hotter than others. Once you have that done, build your set lists (playlist in Windows Media Player lingo) and look into some USB foot pedals to start and stop if clicking the mouse or tapping the space bar is a big deal.

I am also curious about what you meant by "but it's output volume is not enough for our monitor". Don't you have power amps driving your monitors that have volume controls on them?

As far as the audience messing with your stuff, I understand that can be a problem. However, if you set up something like baby gate kind of stuff to make a definite "stage" area, people will not be likely to cross it. Most of the solo and duo type acts I know carry railing in their vans for that very reason. Also, very few of them take laptops out. Of the 5 or 6 I know who do acts like that, they use either an iTouch or a Nano. With the amount of storage on those devices, they never run out of music.