I can feel how disheartening it must be, when sometimes you think you have cracked it with him and then oh dear the kitchen is a mess again, I honestly think we would all feel very down in the dumps about it. It is hard work taking in an older dog who has not been socialised the way we would do it ourselves and to know a dog may well have turned out completely differently if we had had them from an 8 week old pup.
You seem to be doing really well with his socialising, Rough Collies are often quite aloof with strangers, complete opposite to your BC

I guess really the two are like chalk and cheese, but you have worked on him with traffic as well so pat on the back for what you have done.
You have two problems with his toilet training, many owners find that their dogs have intolerances to certain foods infact some dogs almost anything will upset their tums, so you
needto remember by your own admission which foods these are. That problem you can solve.

However as you have stated he also does toileting in the house when solid too, that is a different problem, as Lindsay has already pointed out this could be fireworks, it could be the rain. (Many dogs hate the garden when it rains ) He is obviously on and off sensitive about going outside to toilet, I'm also wondering if he is doing some territory marking here also.
Most dogs, especially when you have more than one have a couple of places in the garden which they frequent, (their own territorial toileting area) male dogs will often pee over a bitches pee too, does your RC use a particular place? If he doesn't the next time he wee's in the house wipe it up with some newspaper or a cloth and place the cloth in a particular part of the garden, take him out and let him scent where his smell is, also do the same with his poo, pop it in a part of the garden and leave it there for a couple of days so that he can mark out his own territory, I've often tried this method with older pups and adolescents and it takes no time for them to encorporate the garden as their territory, and the place to go.
Continue to treat him as a pup with his training with praise and reward, (glad to see you are not chastising him

) he sounds to be pretty stubborn, but understanding why he is doing it will be the first steps to curing him, not knowing if it is territorial or if he is upset about something will be hard to determine, I agree that a behaviourist could well work wonders, but also understand finances can make a difference here, try the territorial placing of his poo and pee and see how that goes over the next 10 days or so.
You must feel very frustrated, I think I would be too, but try to understand there is a reason behind it, behind everything that a dog does, if one thing does not work try another, and the slighest bit of improvement is the way to go and hopefully you can keep building on it.
Good luck! Let us know what happens, and you may well get other helpful hints too.