UKC

iI the absence of enough QD's I trust it is reasonable to use a single biner on a bolt? Most 90's climbing films you see it anyway?

Definitely not best practise.

It's far more likley to unclip than a quickdraw.

If it is the only option available use a screwgate if possible.

Works fine. Good on short routes where you want to minimize rope out if the next fall would come close to the deck. 

On longer routes it causes a lot of drag so best avoided. 

Thanks - yep would be on short routes only and would take care to minimise any drag.

Planning an outing with only 5 draws but have a few extras biners.

You can do it. Assess your own risk, make your own decisions. There are no rules in climbing and plenty of scenarios where less conventional techniques are necessary or even preferable. 

In answer to your question, your nut is more likely to pull out with rope drag, depends on the route and your placement. 

On occasion I have clipped the first bolt with a screwgate (done up) to minimise the potential fall distance so close to the ground. 

Try and plan ahead, take enough draws, but Should you run out, use a screwgate and do it up. 

Worth noting that if the lower off requires a re-thread you will want to prioritise the use of a screwgate here. You may only carry a couple on a route! 

If you look at the bolt side of your QuickDraws you’ll probably find they’re a bit burred and chewed up from the steel bolt hangers digging into the softer aluminium when you fall. 

This will happen with your screwgates too if you fall off, so keep alert, you don’t want a heavily burred screwgate getting into a position where it had textile items moving over it. 
 

Other than that, your probably not going to die using a screwgate rather than a quickdraw. I do this quite often on bolted slate routes for example. 

If you're standing at the bottom of a 30m route with 6 qd's and you think, oh yeah, easy, I'll split all the draws so I can do this route... then no, not a good idea. If you've misjudged the amount of bolts, or just forgotten to take enough draws and you're thinking, If I split this one I can make the anchor then yes, 'generally' fine.