Vic, you are absolutely correct that Barney needs to be 100% comfortable with you examining all of his bits & by continuing to groom him & checking his ears, eyes, mouth etc regularly he will soon build up his trust in you.
If he starts getting all wriggly & won't calm down when you're grooming him I would either distract him by giving him something to chew & continue grooming him- mine love cold (washed) carrots from the fridge - or break up his grooming sessions into smaller chunks. He'll be bored that's all.
What I also do is that anything that I do that the dog may find even slightly unpleasant, such as cleaning their ears, checking between their pads, applying flea treatment, giving them worming tablets -I counterbalance it at the end by ALWAYS giving them a treat.
When they're puppies it's usually a high value food treat such as chicken, & as they get older I'll mix it up - so sometimes a ball game, extra walk, or a food treat. They quickly learn that if they stand still, whatever treatment will be over faster & they can have their treat.
I've recently had my female quite poorly & she's had to have quite a few meds given to her in the form of anti b's, ointment, drops, tablets etc - so much so that even the vet enquired if I was ok administering them all. I didn't have a peep out of her, she stood as good as gold waiting patiently for her treat to arrive at the end of it - & that was 3x's a day for 2 weeks at the beginning.
When he's being naughty I would simply redirect him.
Have you tried training him to sit yet? You should try - you'll be really surprised at how quickly he picks it up. As he's so tiny just remember to keep the training sessions short - say 5 mins. Several shorter sessions a day at this age are much better than one longer one.
Also try rolling a ball just a few feet away (one that fits in his mouth, but make sure he can't swallow it obviously, & don't leave him to chew it), you may need to get him interested in the ball first before you roll it, then as he starts to follow the ball insert a command such as "fetch", as soon as he picks up the ball call him back to you with another command "come", "here", whatever you want, but it's important that you choose your command words & stick to them. Make sure you're crouched down to his level as he arrives back to you, then lots & lots of praise for being the cleverest poopster in the world. All this will help build up the bond between you, which is the key to getting him to do all you ask of him as he won't want to displease you.
I would just have a google before you fork out for a Furminator - I think(but not 100%sure) that they work by removing the undercoat & most long coated GSDs don't have an undercoat. Get a Sebo instead!
As for the nervous breakdowns - wait until he hits the "Kevin" stage! Oh my! You'll be having at least 3 a day!
What does your little Cavi make of him?!