just my $.02.
1. The brewer of the beer in most readings of applicables laws that I've found on line is the person who pitches the yeast. This means if you take home a bucket of sugar water and then pitch the yeast, you are the brewer even if all you did was watch. Additionally wort is wort, but as soon as wort meets yeast, it is beer for most legal purposes.
2. The controlling sections in federal law seem to be these.
§ 25.205 Production.
(a) Any adult may produce beer, without payment of tax, for personal or family use and not for sale. An adult is any individual who is 18 years of age or older. If the locality in which the household is located requires a greater minimum age for the sale of beer to individuals, the adult shall be that age before commencing the production of beer. This exemption does not authorize the production of beer for use contrary to State or local law.
(b) The production of beer per household, without payment of tax, for personal or family use may not exceed:
(1) 200 gallons per calendar year if there are two or more adults residing in the household, or
(2) 100 gallons per calendar year if there is only one adult residing in the household.
(c) Partnerships except as provided in § 25.207, corporations or associations may not produce beer, without payment of tax, for personal or family use.
(Sec. 201, Pub. L. 85-859, 72 Stat. 1334, as amended (26 U.S.C. 5053))
§ 25.206 Removal of beer.
Beer made under § 25.205 may be removed from the premises where made for personal or family use including use at organized affairs, exhibitions or competitions such as homemaker's contests, tastings or judging. Beer removed under this section may not be sold or offered for sale.
§ 25.207 Removal from brewery for personal or family use.
Any adult, as defined in § 25.205, who operates a brewery under this part as an individual owner or in partnership with others, may remove beer from the brewery without payment of tax for personal or family use. The amount of beer removed for each household, without payment of tax, per calendar year may not exceed 100 gallons if there is one adult residing in the household or 200 gallons if there are two or more adults residing in the household. Beer removed in excess of the above limitations will be reported as a taxable removal.
(Sec. 201, Pub. L. 85-859, 72 Stat. 1334, as amended (26 U.S.C. 5053))
Reading over 25.205 we get 'yeah you can home brew' and in 25.206 we get 'yeah it doesn't have to be at your home' and with 25.207 it appears to say 'but you can take it out of the brew location to your home as long as 25.205 and 25.206 are met' HOWEVER not being a lawyer, someone else should vet my understanding.
There was in the original 1979 legislation a clause about only having 30 gallons of HB on premisise in all stages of production (from first pitch to cellared bottle) but this seems to have been removed in later ones. I also notice that they now alow removal for competitions, and that 25.207 seems to have been designed for clubs. Again, your state may impose more limitations.
3. Given the the penalty for going over the limit is to be taxed on beer, and the federal law is that you have to pay the tax first before being able to litigate, the TTB doesn't need to prove you brewed over the limits, it is up to you to prove you didn't. This is basic tax law (since I think 1870s), and effectivly is a presumption guilty until proven innocent.