I use melodeon to accompany songs sometimes, but it's a bit limiting because of only having 2 keys, which may not be suitable for all your songs. Oddly enough I find that I can sing + play, but can't talk + play at all.

Despite allegations to the contrary, it does have a natural punchiness that comes from the push/pull action (that's what makes it a good choice for dance music), but you can override this to some extent by using more chords and drones for accompaniment.

As to whether the melodeon is easy: I think it divides people more than any other instrument. You either take to it or you don't. Try to borrow one and see what you make of it. If you can soon play tunes by ear quite easily, you'll be fine. If you have to write the letter-names on the buttons and can't play without staff notation, it'll be a struggle.

If you really want an instrument mainly for song accompaniment, English concertina would give you the full range of keys and notes that a diatonic instrument doesn't have. Again, try to borrow one and see how it goes. I borrowed one and found I couldn't get on with it at all, whereas with a melodeon the basics all fell into place very quickly.

Good luck!
Marje