I visited Nepal Mach 28 through April 24 this year. I trekked at the Annapurna curcuit for about 20 days.May be some of my experiences can answer your questions

My English is poor. In Nepal they call trekking. I hope trekking and hiking mean the same thing. At Annapurna curcuit, It is possible for you to be independent trekker ( no guide, no porter ) I was independent treker and there were about 50-60 trekkers walking in the same direction with me each day. Days by days pass we become friends.

I think the average age of trekkers is about 30 years. Only 4-5 trekkers are ages more than 60 years. There was a German guy ages more than 60 and he was carrying his backpack ( did not hire porter ) during the trek.The most difficult part of Annapurna curcuit is Thorongla Pass which is 5400 metres above the sea level. I recommend you hire porter at $US 8/day.

About narrow path, There are narrow paths all the way. The width of the narrow path is about 2-3 metres. At narrow patchs,caravan of 30-50donkeys, horses and yaks with huge goods on their backs always walk on the opposite direction of you ( and same direction too). At this situation, I recommend you stay at mountain wall side and let these donkeys stay at hell side.Normally these animal walk in queue. If one donkey does not walk in queue,raise you hand with hikking stick up high before this donkey come near you.

About the suspension bridges,my trek has about 30-40 suspension bridges. These bridges are strong and safe. Donkeys caravan also use these bridges.

Please read Nepal travel guide book and you can find the answers

I hope this could help Kai, in Bangkok