Increasing your nerve bar with merits does not increase your gained experience. The main advantage of adding to your nerve bar with merits is so that you can fly farther without completely filling your nerve bar.
Increasing your nerve bar by doing crimes is an indicator of how much crime experience you've gained so far. When someone asks for your natural nerve bar (NNB) they are referring to the size of your nerve bar without any merits or faction specials added to it. For example, if your nerve bar is 32 but has +3 from your faction and +4 from merits, then your NNB is actually 25 (32 minus 3 minus 4). The "25" indicates how experienced you are in crimes.
Increasing your nerve bar with crimes always increases it by 5, and so your NNB will always be a multiple of 5 (15,20,25,30...)
For your second question, here is a list of all of the candy (happiness), alcohol (nerve) and energy drinks (energy) and how much you gain from each:
https://www.torn.com/wiki/Item_CooldownThe "higher gain" items have the same cooldown as "lower gain" items but are much more expensive.
Beer is quite cheap, so even if you cannot afford anything else you should keep your cooldown bar full by using beer. Note that all of the items marked "cannot buy" in the list were giveaways in special events (and goodie bags) and can be bought at the bazaar if you have a lot of money.
If you have not yet played the "Torn Lite" facebook app, I suggest you do that and get the free goodie bag with some nice drinks in it. Let me know if you need info on the Torn Lite app.