It's really hard when you are just starting out to decide which advice suits your situation, and it's equally hard for us, who are not with you and your ewes and lambs, to give exactly the right advice for the situation you are in based on your description of what has and is happening.
It sounds to me as though the lambs have probably had enough 'squirty' vitamin supplements now. I wouldn't be using the 'boost' type products as a routine, just one shot if the lamb is fading. If you are wanting to give a routine vits + electrolytes because it's scouring and/or has watery mouth so it's not taking (or shouldn't have) milk, then I'd use PSF or Rehydion.
Reading the datasheet, Spectam is one shot per lamb only. It sounds as though it's used only as a preventative not as a treatment. We have Oraject on hand which is one shot at birth as a preventative but which can then also be used as a treatment if the lambs develop scouring and/or watery mouth. It's easily justifiable with a commercial flock of 200+ ewes, but harder when you just have the two! Maybe next year you can get your local friendly farmer to give you the remains in his/her last bottle...
For watery mouth I would give Pen & Strep 1ml per day for up to 3 days, maybe up to a week. If it has really got watery mouth I would not be trying to get milk into it once it's had colostrum a couple of times, I would be using Rehydion in water or very weak milk until it starts to get its appetite back, then continue the Rehydion in increasingly milky water until it's recovered and on full milk.
Reading Norbrook's website about Life-Aid, I cannot see that it can be added to milk, no. But as you say, you don't want to buy more products if you already have something you can use, so for a lamb with watery mouth then use the Life-Aid you have, in water, until it gets its appetite for milk back.
Im not sure who told you to bottle feed. I always tube if I want a lamb to remain with its dam. There shouldnt be any "forcing" involved with tube feeding.
Well I prefer the bottle but not to the extent that you're forcing the lamb to drink. If it won't take the bottle, I would, as you suggest, smudger, tube 60ml in and then see if it wants some bottle as well.
Sometimes if the lamb is not keen on the teat it can help to tip the ewe up and get the lamb latched on that way - so long as mum doesn't kick her in the face. Once lambie has the idea it will usually find and use the teat wherever it is but some of them really are dumb about getting the hang of it at first.
The other thing that can be worth trying with the bottle is different teats. The thick hard plastic teats are not, in my experience, very palatable to newborn lambs. I usually start with the standard 'non vac' bottle with the very soft translucent yellow teat, with quite a small hole. Some lambs just don't like it, it's too squidgy I think, so then I try the soft red teats that screw on to fizzy drinks bottles - some lambs take to that straight away then you can switch them onto the other teats in a day or two if they are still needing bottle.
And a small cautionary note on the tactic of using a dog to stimulate the maternal instinct - with some hill sheep especially, the combined stress of birth, suckling (or not) a lamb they aren't liking, being penned up and lots of human intervention, coupled with a dog in close proximity can just make them flip. I have had Swaledale ewes, cross with the dog that is outside their pen, beat up (with their horns) the lamb that is in the pen with them. So it can be worth a try - but be ready to rescue the lamb and get the dog out of there, pronto.
Hang in there smudger - quite often they do quite suddenly rally. I have my fingers crossed for you.