The Start Master trick only gets you essential DC, not AC, and is of limited use. It gets you OAT, cabin and duct temp indications, left windscreen wiper, both engine displays (not much use in this case), engine oil indicators, and feed tank and wing tank fuel quantity.
The transponder #1 is powered by ESS AC, so you wouldn't have that.
As for descent rate, yes you can get high rates of descent, but you need green hydraulics for the airbrake which would require a running engine.
Book figures for glide is 2.5:1 clean and 1:1 with gear down.