Yes, I would imagine that the use of the FYM (farmyard manure) in the compost would compensate somewhat for the cost of forage and bedding. We aim to return to the ground that which came from the ground, so in a year when we buy in all the hay and straw we are happy for a lot of the FYM to go to the compost, but if, like last year, we made our own hay/hayalge, we would then apportion the FYM accordingly, trying to put back on the field the dung produced from the grass taken off the field. It's very inexact, of course, because we take calves and lambs away for meat, fleeces for spinning and other crafts, and surplus milk (the cows feed their own calves, we take the spare). And we feed very little bought-in feed, so we are, over time, depleting the ground.
In terms of overwintering inside, cattle need to have the rain kept off them but the next most important thing is ventilation. Ours have a yard to loaf in, an al fresco hay feeder with a rain hat, and a very well-ventilated strawed pen which they are free to enter and leave at will. The whole thing was built with ventilation the foremost consideration. If you can't give them something that airy then the worry is pneumonia. You won't hurt them shaving a strip along their backs, and you may find they stop growing so much hair once they get used to being in for winter.
I would guess that Belties are probably similar to Dexters in terms of need for forage. They are larger but they really are exceptionally good "do-ers". I haven't kept Galloways myself - not for want of trying - but on the upland Cumbrian farm we had Blue Greys, which are Galloway x Whitebred Shorthorn. Doris was in one winter and got fat as a tunky pig sharing the same rations (just hay and silage, no cake) as the other sucklers (mainly Angus crosses.)
I would think 3 stirks would probably be pushing it yes, on 10 acres when there are sheep and other things going on as well. But you might get away with it if you are wintering inside.
Yes, TB reactors have to be put down. And it happens :/
The rule of thumb is a cow needs as much land for hay as she needs grass under her feet in summer. We like to make at 3-4 acres of hay if we can and that will pretty much do the 2 cows, 2 ponies (who get very little) and the 18 or so sheep (who get about the same as the 2 ponies.)
It's a perennial question, is it worth doing your own hay. We are blessed with wonderful local contractors who don't mind our small acreages and don't charge too much for cutting, turning, rowing up and baling, and can wrap small bales if it's not possible to make good hay that year. (Which happens often :/)
Last year we made 233 bales, and had to wrap the lot. Cost £3.70 per bale to make but was wonderful stuff; equivalent to buy would have been £6/bale. In fact, because we prefer hay to haylage for the ponies and sheep, and also for the cattle to have a mix of some hay and some haylage, what we did was sell some of the haylage and buy hay in with the money. But almost all the buyers were horsey folk and wanted it delivered 10-15 bales at a time, so it was quite a lot of work - and we needed our sheep trailer.
Cotswolds are longwools and I would say therefore not a sheep for novices unless you are very 'into' your sheep. Longwool takes a lot of looking after, very prone to flystrike. Shetlands and Manxes are naturally resistant to strike, I don't know about Jacobs.
Dogs can be a heck of a nuisance, but it's usually far worse where there are lambs. So if you aren't breeding, and therefore only have adult sheep, that would help. Best sheep for standing up to dogs that I know are Manxes and Swaledales; they are both horned and both cracking conservation grazers. (I have no firsthand experience of Jacobs.) However, it is usually the ewes that stand up to the dogs, with their maternal protective instincts. Wethers can be a bit dopey ;p
Electric fence would have to be two or three wire strands (can't use the netting style with horned sheep) and takes a lot of looking after. Also, it only deters dogs who know about it, so whilst the local dogs would learn to respect it, visitors' dogs might be through it and chasing the sheep before they realise it stung them.