Hello Rohit-

I can only speak with experience to #4:  A 6'5" rower can row comfortably from the NE Dory's bow station without any thwart modification.  I am 6'5", and it is my preferred position when rowing tandem.

My initial reaction to #2 is that the beam is too narrow to allow two rowers to handle sweeps without some kind of outrigger arrangement.  The inboard oar length seems like it would be inadequate.  But I've never tried it.

Re. #6:  Consider using some 60"+ boards laid across (perpendicular to) the vehicle's existing runners.  If everything else is sized, situated, and secured appropriately, they should provide the necessary support.

 I love my NE Dory.  HOWEVER, for a short trip on relatively protected waters (waves less than 2 feet, with a protected place to go if conditions change) and with no more than a lifejcaket and waterbottle and sandwich each, I think 4 adults (independent of weight) is absolute max comfortable (and safe) capacity. Anything more than the conditions above, and I'd be more comfortable with only 3 in the boat. 

With 5-6 people in the boat, gunwales are nearer to the water, and you never know who's going to lean the wrong way at the wrong time - especially with many sets of oars flailing about.  While the boat can be recovered if swamped in open water, it is tricky.  In any significant waves, with many people swimming around, things could get dicey in a hurry.

Trying to keep more than two people rowing in unison is probably more effort than it is worth.

I've got more than 50 years on the water now, and my head is still above the waves, but that is because I try not to tempt Davy Jones, and every time I ever did, I came back a little more conservative than I was before.

Based on the questions you are asking, and with tongue only slightly in cheek, I refer to the movie quote:

"You're gonna need a bigger boat!" 

Or perhaps a  better idea - how about you get your team TWO dories!  That sound like a great idea to me (and CLC, too...).

Or perhaps the Team Dory?  Or the lovely Pilot Gig?  I'd love an excuse to build either one!

Thanks WildlifeBio, Bubblehead:

That's very helpful information. I did consider the Team Dory, the Pilot Gig and St Ayles Skiff but they don't fit in my 21' garage. My house is 7 min walk from the water, and I have a tandem Alden 18 that I drag back and forth on a dolly. I was looking for an additional boat that

1. I can manhandle on the dolly myself (<150lb?)

2. I can take the most number of friends on and 

3. has the most room for relaxing.

So right now it still looks like the NE dory, which has a weight:passangers ratio of 100lb:4 is the best non-inflatable boat I can get.

Rohit

   Have you considered the SW Dory instead?  While the payload is the same as the NE Dory, the boat is significantly bigger, more comfortable, safer with built in floatation (especially for open water) and places to store gear.  I'm not suggesting that you could seat rowers side by side, but it sounds like it would suit your needs much better while still buildable and storable in your 21' garage space.  

   Have you considered the SW Dory instead?  While the payload is the same as the NE Dory, the boat is significantly bigger, more comfortable, safer with built in floatation (especially for open water) and places to store gear.  I'm not suggesting that you could seat rowers side by side, but it sounds like it would suit your needs much better while still buildable and storable in your 21' garage space.  

I have a NED. I'll reply where I have personal experience.

2. I doubt it. At least not with the standard oar setup.

5. I cartop my Dory. First I used 2x4s, then I bought 60in aluminium "square tube" from the DIY store and bungeed them to to the OEM cross bars. Might go back to wood though. Don't like the way the alu fatigues. I also use a Reese towpower canoe loader and a Mallone dolly strapped to the bow to help load and unload. The hitch mount also takes almost 50 per cent of the load. 

Regarding manhandling the boat from your garage to the shore, you probably need a bigger dolly than the standard canoe / kayak version. The hull is very wide at the middle. I have a canoe dolly that I strap to the bow and move the boat around "wheel barrow style". 

I have a 20ft garage and it fits nicely.

PS: a lot of people ask about cartopping the dory and I'm planning to do a YouTube vid sometime this summer to show how I do it.