I just begun Heartache by the number and You can depend on me sidequests. I must deliver an invoice to Dr Hilbern in Fort McCaran but I have already completed There stands the Grass, so the only dialogue option I get when I speak to him is "Time for me to go" and can't give him the invoice. Does this mean I can't go any further and won't be able to recruit Cass?

No. Just get your speech or barter or whatever high enough and go recruit Cass at the Mojave Outpost bar.