I'd say it is too late to plant grass seed now. I'd leave it to the spring.

The usual time to cut back lilac hard is mid autumn so it is arguably a bit late now although they are very tough so you could probably get away with it. If you very hard prune it ,it is unlikely to flower again for at least 2 years.

I hard pruned some very large lilacs on my property down to about 3 feet tall just after flowering about 3 years ago and they flowered well this year

This photo shows the lilacs this spring. The ones in the centre with a small amount of flower are the ones I hard pruned. The one on the right with much more flower was not pruned and the difference even 3 years later is obvious.

http://www.flickr.com/photos/31559373@N00/4585271668/

Grass seed wants a temp of 5 degrees, consistent, so, yep, a bit late. Fluctuating night and day temps. will stop germination. As A/man says, lilac will take it. Be brave, but forget flower for a few years. Phil

you have 2 windows of opertunity with cutting your lilac back and one is late autumn and the othere is very early spring thow like anchorman sais its quite a tuff shrub .

But be warned - hard pruning lilac often encourages masses of sucker growth...

your more than welcome masie x .