The biggest problem is you're trying to train for something that is inherently chaotic and fluid, but you're stuck in a static situation (no partner). You won't get a lot crossover on the actual clinch work, but you can work the knees.
If you have access to a bubba dummy, you could work your clinch on that. It's a fairly decent representation of a person. Really work on folding the dummies chin to it's chest and "choking" it by squeezing those elbows. The downside though being that it's pretty stationary and doesn't fight back.
Work those knees on the dummy and clinched up heavy bag.
Other than that, you really need a partner to train clinch work and pummeling. Maybe ask around at the gym and see if anybody is interested in getting together outside of class time to work out.
Stretching wise, check out Tom Kurz books and vids. It really helped out my flexibility and kicking back in the day. The guys gotta be close to his 60's now and can still do full splits with no warmup, throw kicks over his own head, etc.
YMMV