Oh.. the mayhem! We've been there too, when we first started out with Angus cattle many years ago. Trying to treat pinkeye with no crush, and the cows were WILD! Luckily back then we had no children, but it was still exhausting, so I really do empathise with you.
Okay, so, Plan A didn't work, we'll switch to Plan B!
This may not work, depending on how wild the cattle are..
Can you get you entice her (possibly need to get all of them just to get her
) into the cattle yards? Do they know the rattle of a bucket? Don't rush them, get someone (without a baby on their back!) to get the leader interested in what's in the bucket or a biscuit of lucerne hay, and then walk up to the yards, and just leave the bucket in plain sight. Call them up, but otherwise just leave them alone. IF they go into the yards, then close the gate. I find cattle can be intensely curious creatures, and sometimes its their curiousity, not necessarily their bellies that get them caught!
If you can get them in, then try and get all the others out, except for the cow and her calf. Then, remove the panels, one at a time, and enclose it in around her, and her calf, until she's in standing room only. Through the bars, have a good feel of her udder, and as said previously, try and express some, for taste etc. which will give an indication of what you're dealing with. Fingers crossed its just engorgement and cracked teats.
Regarding the "Today" antibiotic, have you ever taken a dog or any thing to a vet around you? If so, ring that vet. Say, with confidence in your voice... "I have a cow with mastitis, I need four tubes of Today to treat her, do you have them in stock?". There are "rules" and then there are "rules", often, if you are "on the books" for the vet, they are more comfortable applying the second set of rules, and give the antibiotic without a on farm visit. Alternatively, you can get back to the first vet, and say, exactly the same thing, and tell him/her, that an onfarm visit just won't happen, but you're certain its mastitis, and need it treated. Nothing ventured nothing gained. Otherwise go to the nearest dairy farm that you have around you, and ask to buy some of their's, they will undoubtedly have some on hand.
Otherwise, try emailing/pm'ing Tam Peirce (on this site) who is down in Victoria, and see if there's any extra floating around that can be posted to you!
Oh wait, I just rethought something... Today is for lactating cows, and will require you to treat more than once, because the active ingredient is less concentrated, due to milk withholding. "Tomorrow" is for dry cow teat infusions, and that's what you give when you're going to dry off the cow, or the cow has mastitis. Its only one dose that way, and stays in the udder for longer, so you don't have to do any follow ups. I'd be doing that one, even if she's still got the calf on her, you don't want to be dealing with her more than necessary.
Okay, this is getting a bit disjointed now, I'm sorry.
Yes, the calf is old enough to pull him/her off, and bucket feed, although, if you can't catch the cow, then you won't be able to milk her out to get the milk! You could always buy some raw milk from a local dairy, and bucket feed him or get some milk replacer, however, didn't you say that the calf is stealing milk from other cows, until his mum sees and gets him busted? If that's the case, you may just need to separate her, and leave him to his own good senses! To transition from milk to feed, buy some calf pellets and soak them in the milk in the bucket. The calf will start to drink/eat them. Cow teats heal up suprisingly quickly, so you may be able to give her some rest, put some salve on her teats to aide healing, and bucket or foster feed the calf for a few days, and then let him back on her and see what damage he does. If too much damage, pull him off permanently.
Beef cattle, are much hardier than dairy cattle and they naturally produce less milk anyway, because they put their energy into their body, rather than their milk (hence the "bony" jersey's you see in the lush paddocks!).
If her udder is already shrinking, yes that is a good sign that she's already making less milk. If you can keep an eye on her, and make sure she doesn't start acting "off", then if you can't yard her and have a feel, well, really, watching her is the best you can do! Ultimately, you can only do what you are capable of, its not always possible to do what is "best" for the animal, simply because they simply make it impossible! You can only do what you can do, and just hope that its good enough.
Let me know how you get on