It seems almost callous to start unpicking the etiquette of tonight's deeply moving and sad episode... but here we go...
Clocks
When valet Thomas was showing the new apple of his eye how to wind up the grandfather clock, he closed the glass door to the clock face with his fingers all over the glass!! How careless, Thomas! Now one of the maids is going to have to come and wipe away those finger marks. The valet was clearly letting his fantasies run away with him.
Fork? Fork and Spoon?
Tonight's dining scene was during pudding (American readers: pudding is what you lot perversely decided to call 'dessert'.) Pudding is correctly eaten with a fork and spoon, unless one is a young child learning to eat, where just a spoon can be used. However Lady Mary was using just a fork. This is actually permissible. When the pudding is a tart, it can be eaten with a fork alone (upturned and in the right hand).
On the menu tonight at Downton was such a tart, and so Lady Mary may have been using a fork, whereas others were using both a fork and spoon. Either method is correct, what is not permitted (for adults) is the use of just a spoon. Also, if one decides to use both a fork and spoon to tackle the pudding, one cannot then place the spoon to one side and continue with just a fork. You make your choice and then stick to it!
Today, restaurants have decided that they can save money and space in the dishwashers by only setting a spoon for puddings (I always ask for a fork and put up with the filthy looks). Naturally, I do not condone this practice as the role of the fork when eating pudding is to push what you have just cut with the side of the spoon onto the spoon. By not having a fork one is forced to use the fingers - which can get messy... as well as looking awful.
Glasses
There's no get out clause of blaming a lack of footmen this time, Carson, but you really should have cleared the glasses from the preceding courses. We had Lord Grantham drinking sherry with the pudding in one shot! That will not do. When a full complement of staff is available, glasses are cleared after use (i.e., the sherry glass for the soup course gets removed just after the empty soup plates, etc). Lots of glasses were still on the table at each setting during the scene.
Kissy, kissy
There was a fair bit of social kissing in tonight's episode. Note that the modern practice of a kiss for each cheek was not in play back in the 1920s. For the British then (and some now) two kisses was far too emotional and the reserve of Continental types. Today, the older generations prefer to stick to just one kiss, whereas younger generations will opt for the European two-kiss variety. Either way, ladies should proffer their checks (so, men, if a cheek is not presented, you do not kiss!). In Britain ladies proffer their left cheek (and then right cheek, if two).
Teacup handles
Ethel could be excused for falling out of practice with serving a cup of tea, but note that Mrs Crawley had to push the teacup around so that the handle was pointing to the right, not the left as it was served to her. Teacups (now and then) should be set/presented with the handle facing right, as most people are right handed.