1) Say, if I at the point of the 5th year have only 28 months of physical presence. Then I can simply wait 2 more months and apply then no?
Not necessarily. The 5 years is a sliding window. If you wait 2 more months in the US, you would add those 2 months of presence, but the start of the 5-year window would slide forward by 2 months, making you lose whatever days of presence you had just over 5 years ago. As a result, waiting 2 months could result in zero or very little net gain. Some people find themselves having to wait over a year just to gain a month.
But yes, if you fall short of the physical presence requirement, you just have to wait long enough to accumulate the 913 days, whether that is 2 extra months or 20 extra months.
2) How long one has to stay for the first trip in the U.S. in order to start counting?
It depends on how long the second trip is. A 2 month stay inside the US followed by a 7 month trip outside is likely to result in the count starting from the end of the 7 month trip. But a 1 month stay in the US followed by a 1 week trip overseas followed by a 4 months stay in the US is almost surely going to result in the count starting from the beginning of that initial 1 month stay.
But again, remember they only look back 5 years, so the count will never start more than 5 years before your application submission date.
Ultimately, continuous residence is a subjective decision at the discretion of the interviewer (and his/her supervisor), so there is no exact formula* for determining whether you break continuous residence or not. At the end of the day, what they try to do is look at your travel pattern and ties to the US and determine the answer to the question "has the US been your primary residence for the past 5 years?"
*other than obvious extremes like never leaving the US at all, or spending over 12 consecutive months outside the US without N-470