Re: Back injury and starting aikido?
Tai Chi is a great alternative.
The thing to keep in mind is that aikido is a paired exercise, so there's a certain amount of (usually positive) pressure to keep up with your partner. I wouldn't train with a back problem (and work not to train when I am injured generally) because I am a fairly competitive person. It's hard for me not to get lost in working to match my partner's competence, speed, power, ability, whatever.
This is of course something that I am working on, but it's a long row to hoe. I don't trust myself to privilege my own body over the experience of training when I know I have an injury.