1) in theory yes.. in practice .. depends on the bid landscape in play and ends up often not being cheaper as other advertisers bid higher on exact compared to broad or phrase
2) exact should take precedence over phrase or broad but yes bid dumb enough and up broad or phrase match ads will go
3) depends who you speak to, some separate based on matching and have exact, phrase and maybe even broad match adgroups for same product/kewords others lump the lot in together, it's probably best to have the exact matches in one, and phrase in another on high volume / value traffic in the long run.. basically you want the most relevant adgroup you can to the adcopy, keywords and landing page
with adwords what happens and what should happen are two different things lately so just keep thinking "what's the most relevant - highest quality adgroup I can create" with regard to adcopy, keywords, keyword matching, CTR, negatives and landing page etc... trim out zero impression and any silly £5 minimum bid keywords regularly, keep the adgroup running with as many live keywords as you can and you won't go far wrong.
course even then you'll be at the whim of the magic £5 bid request.. where keywords not found on the page may be fine at 10p and the main keyword "x" on your site named find-cheapest-keyword-x.co.uk which is on the page 20 times may be £5 a click due to "landing page quality"
