Not sure why you think that the separate USB cable might take longer to charge? What matters is that the source you use to charge must meet the minimum current requirement of the charger, which is 500 mA, as specified in the manual. A 5V output delivering 0.5A current is only a 2.5 W draw, very minimal. Even the smallest 5V power sources usually deliver at least 1A of current anyway.
For this small amount of current, most USB cables will suffice and will not cause the charging time to be slower than specified.
As to what happens at the end of life for the battery, yes, usually you will start noticing that your hearing aids won’t last 20 hours with a full charge anymore toward the end of its life. Maybe you’ll get 16 hours out of a full charge by then (only a guess here, could be more, could be less than this). So you’ll have plenty of time to send them in for new battery replacement toward the end of its shelf life.
Normally hearing aids carry a 3 year warranty. Usually, many HCPs are willing to do a warranty claim for you close to the end of your warranty period to get you a new pair of aids, even if your old ones are not broken yet. This practice may depend on the HCP, but mine did that for me on my OPN 1. So if yours will do it for you, you should ask to have new Oticon Intent aids replacement before the 3 years is up, thereby having your built-in batterie refreshed automatically as a result for another 5 years, totally 8 years before you have to worry about battery replacement again (or upgrade to a new generation of aids by then).
The screenshot below is from the Intent charger manual about the power requirement: