Whoa I go away for one day and the thread gets huge!
Firstly, thank you all for your responses. It is always good to get the perspective of a cross section of people, to help realign my own perspective if nothing else. I would also like to say thank you for the suggestions, there were a few in there that we haven't tried so I will give them some thought on how to apply them.
I would like to clarify a few things and answer a few of the points raised, but with 8 pages of replies since I last read the thread, I'm afraid I can't remember who raised which one so please accept my apologies for not crediting who I am responding to.
The other mum is also refusing to take DS2 as well. Her reason for this is that this week, he was late out of the changing room as well. That was because he was trying to chivvy his brother on but I can see that from her POV it was just that both of them took an age. She is still expecting the lift for her daughter from the normal meetings.
I can't drive because a) I can't afford lessons or the upkeep of a car even if I passed my test and b) I have to take pain killers that I can't drive on for a chronic joint problem.
The lifts were a reciprocal arrangement, in which my responsibility was to arrange transport for one journey. I did this by arranging for my mother to do that run on my behalf.
The other mother was aware of DS1's limitations and abilities when she agreed to the lift (which was thought up by her and one of the other mums and suggested to me, I had assumed lift share would be split as it is for normal meetings) and this included her knowing of his difficulty in staying on track WRT changing.
My mother has another commitment which means she cant take, wait and collect on canoeing days. She has agreed to pick up if the other mother agrees to still take the DC to canoeing as shenwould be back in time for collection. The difference (to me, and I accept that others may see it differently) is that the other mum is already staying and helping out, something she has chosen to do herself, whereas my mother would be sitting in the car waiting. The scouts don't need the extra help from an occasional helper (we are very fortunate in having a lot of parent helpers) and she wouldn't be able to commit to helping more than once every few weeks so it isn't worth it really for the scouts to get her fully checked out and CRB'd for those odd times, so she would just be sitting for 90 minutes in the car. TBH it seems reasonable to me to say she is unprepared to do that every week. She cannot spare more than the odd occasion to help because she already runs a local Guide group and so that has to take priority over helping out with the scouts, especially as she is desperately short handed. For some reason, people are falling over themselves to help with scouts but not guides round here.
For the same reason that a leader couldn't be in the changing room to chivvy DS, nor can I. I am a woman, he is in the boys changing room. I am sure there would be a kick off if someone came here and said they had just found out that some strange woman was in the changing room with her DS when he was changing and drying himself after a water based activity. All I could do is knock on the door and shout through to hurry him along, assuming that could be heard through the cacophony that comes from a room full of hyped up 10-15 year old boys parking about and getting changed 
The other mother isn't some random stranger i barely know, she lives over the road. The 5 who go together all live within 50 yards of each other and have always lift shared for cubs etc. This mother is relatively new to the lift share as her DD didn't go to cubs.
AFAIK the issue isn't anything to do with other commitments or pressing engagements as every week so far she has spent 15 minutes standing outside talking to another of the mums whose DD goes to canoeing, before either going home or joining her for a drink.
DS2 is a year younger than his brother, so 10. He helps to a certain extent but like many siblings they also bicker so there is only so much it is reasonable to expect of him. Helping keep his brother on track where possible, yes. Making him responsible for someone else, no. He is still a child and IMO caring duties are the responsibility of adults as much as possible.
Having DS1 get out of the water even earlier than he already does wouldn't help as it isn't so much capability of dressing himself that is an issue as keeping on track with what he is meant to be doing. If he was in the changing room by himself he would just sit. If there are other people there he is more likely to click on that he is meant to be doing the same as them so cop on with it.
DS1 already takes clothes that are easy to change into, so a loose t shirt, zip up hoody and tracksuit trousers. The sort of thing you take to change into after swimming if you are in a hurry and don't care what you look like I completely agree that wetsuits can be a PITA to change out of. We started using it as he had taken too long changing out of wet trousers and tshirt so I thought that only having to take off one item might be a way of minimising how long it took. It obviously isn't working and in fact looks to be making it worse as it is unfamiliar, so if he has the opportunity to go back, we will revert to trousers and t shirt again.
The DC's dad is not on the scene. I have a DP but he lives a long way away and we only see him on odd weekends. He works during the week so is never here on the midweek canoeing days. This is why my mother steps in on occasions where a lift or an extra pair of hands is needed.
I'm not entirely sure of the relevance of what I do during the time they are at canoeing, since I have explained in previous posts that I can't either go along as well due to lack of space or to drive myself, but it has been mentioned a couple of times so as people have taken the time to reply, so I will respond. Canoeing night is the same night that I have a meeting for the fundraising committee for the local community centre I am on. That meets every 3 weeks and as we are approaching the summer which is a key time for fundraising events, isn't something i can regularly miss. Especially as the next big event is a fete which I am organising which happens in 5 weeks time. The other 2 weeks tends to be spent either catching up on study or resting as my condition dictates. Unfortunately I have to limit the amount that I do and take time out to rest up regularly or the pain gets too bad for me to continue with basic day to day tasks. With 2 DC, time to rest is obviously limited so I grab it where I can.
There are probably around 20 sessions of canoeing left in the season, but my DC will only be going for another 6 of them as we are moving out of the area at the start of the summer holidays so I guess that would be classed as a handful of sessions where DS1 could be a potential inconvenience.
Sorry if I have missed any salient points, there was a lot of discussion to take in and remember and my apologies for the random order of the points above. I typed them out as I remembered them which may well not be the order they came up in. If I have missed anything, please feel free to poke me again about them and I'll reply when I next get a chance to get online.