Before COVID Haunted Mansion had stopped allowing ECVs into the indoor portion of the queue, but a park provided wheelchair was available, but you had to ask and sometimes get a lead involved to get it. I am happy to report that at least on July 17th, they were allowing ECVs through the entire queue, you could take them straight up to the ride vehicle if need be. There were similar issues with Rise of the Resistance, which have also been corrected.
The only attraction that might still be an issue is Star Tours, but again there would be a park provided wheelchair.
I will try to go through each attraction and what options there are (note policies can change, but always tell them you need to take the ECV as far as possible and if you need to transfer to a wheelchair, you need to take it up to the ride vehicle). Note these are specifically for Disneyland, Disney World.
Tiki Room - There is a ramp to the left, you can remain in a chair or ECV. If it is a busy day, ask if you can leave via the entrance, as it can be difficult to get across the line for the Dole whip place at the exit.
Jungle Cruise - A return time will be provided, you will return to the exit of the attraction. You can remain in the
scooter OR transfer, taking the scooter right up to the boat. If you transfer, they have boxes they will out in for you to make stepping in and out easier. In general, CMs aren't allowed to provide any assistance with transferring, this is one of the exceptions where they will provide a steadying hand and a little assistance up.
Indiana Jones - You will enter through the designated queue (which varies) after a return time. You can take the ECV all the way up to the ride vehicle. Be aware that In some circumstances the vehicle may come back to the side opposite of where your mobility device is, especially if they didn't have you use the elevator. Should this occur, DO NOT get off the ride vehicle, inform the ride attendant, generally they will have you ride a second time to get back on the correct side.
Tarzan's Treehouse, there is an entrance near Pirates that lets you explore the base of the tree, not too exciting, but is an option.
Pirates - You will be given a return time, when you come back, you will enter through the exit and can take the ECV all the way up to the boat.
Train (all stations except Main Street), you will use the designated wheelchair entrance, which is usually the same as one of the exits and you can remain in your ECV; however, you may have to wait for more than one train and at present they only seem to be running one train, which means 20 to.30.minutes. if you are making a round trip, let them know and they will advise where the ECV can be parked.
Haunted Mansion - presently you can take an ECV all the way to the ride vehicle, if this changes, there will be a wheelchair available, but you have to ask for it.
Rafts to Tom Sawyer's Island - Can remain in ECV and most of the island is accessible, but some trails.and caverns aren't.
Davy Crockett Explorer Canoes - There is a lift, but it is small, so you may need to leave the ECV at the top, just let the CMs know you can't stand long and they can tell you when you should come down.
Big Thunder - you will return to the exit after a return time and can take an ECV all the way to the ride vehicle. Ask for the transfer row if you need to avoid the step down, otherwise ask for the back row.
Splash Mountain - You will return to the exit after a return time and can go all the way to the ride vehicle.
Winnie the Pooh - most mobility devices fit through the standard queue (there are a couple within the rules.for the park that don't and the Cans are usually good at spotting those and sending you to the exit), so you can take it through the queue. Most ECVs can fit on the accessible vehicle. Basically a Victory 10 or smaller (3 wheel will be easier), the current park rentals should also fit.
Rise and Smugglers run - can take the ECV all the way through the queue up to the ride vehicles.
Fantasyland Dark Rides - They presently.ask you to park outside the exit of the attraction if possible and walk in through the exit after a return time. Virtually no wait, but if the distance will be too much, let the CM outside the exit know you can transfer, but can't walk that far and they will advise. Generally, they will then let you take the ECV right up to the ride vehicle, they are just trying to minimize the number being brought up there due to limited space.
Carousel - Can remain in ECV
Casey Jr./Storybook Land Canal Boats/Alice in Wonderland/Teacups/Dumbo/Matterhorn - Can take ECV right up to ride vehicles. Some need to go through exit
Small world - can remain in ECV (most ECVs)
Roger Rabbit - can take ECV right up to ride vehicle, exact procedure varies with line length and FP queue use.
Gadgets Go Coaster - enter through exit and can take ECV up to ride vehicle.
Finding Nemo Subs - can take ECV up to ride vehicle, but there are stairs to go down and the alternative experience is honestly a lot more comfortable.
Autopia - can take ECV right up to ride vehicle. Procedure depends on if the lift is working or not.
Launch Bay - ECVs allowed in, but kind of pointless with no meet and greets.
Space Mountain - enter through the exit, can take ECV right up to a ride vehicle that is pulled off to the side allowing an unlimited amount of time for transferring.
Star Tours - Need to transfer to a wheelchair that can be brought right up to the ride vehicle.
Astro Orbiter - ECV can be taken right up to ride vehicle.
Buzz - Can accommodate most ECVs, but may require a transfer if not.
Great moments with Mr Lincoln/Main Street Cinema - can remain in ECV
Animation Academy/Philharmagic - Can Remain in ECV
Monsters Inc - Can accommodate most ECVs smaller than the old 4 wheel park scooters
Guardians/Spider Man - Can take ECV right up to ride vehicle
All cars land attractions - Take ECV up to the ride vehicle, but requires transfer.
Toy Story - Requires transfer to manual wheelchair or ride vehicle, but can take ECV up to ride vehicle.
Carousel - May remain in ECV
Incredicoaster - can take ECV up to ride vehicle
Ferris Wheel - Can remain in ECV (I do not recommend this on the swinging gondolas)
Jumping Jellyfish - can take ECV right up to ride
Little Mermaid - can accommodate some ECVs, otherwise ECV can be brought up to ride vehicle
Grizzly River Run - use the pull of for mobility devices and can take ECV right up to ride vehicle.
There are portions of Grizzly Creek Challenge trail that are fully accessible.
Soarin - Can take ECV right up to ride vehicle, if you can't walk the distance they want you to after parking it, let the CM know and they will park it for you.
As to if you will need return times, it will vary based on the attraction, length of line, etc., So just ask.
And in all cases, if your needs are not being met, just tell the CM and they will do what they can to find a solution, but keep it concise, like I can transfer, but not stand for more than 3 to 5 minutes.
Also, some attractions will ask if you can handle stairs if it breaks down, answering no will not usually prevent you from riding, but may result in a longer wait.