This is what I was concerned about with dogs and small children/babies who were born into the covid world, or within' a year or so before...depending if dog, or child.
While this is less about the covid world than it is you not having had a van to take your power chair out in, I think that it's very similar.
You will probably want to desensitize her. Here are a couple of thoughts.
1 - Go out even if you don't need to. Maybe Belinda would agree to take you out 2-3 days a week, even if it's just around a couple of blocks so that Arwen sees you getting ready to go, going, and returning.
You can also crate her, and go out onto the front porch on the in between days. Sit out for a short while, come back in and go through the uncrating routine with as little fanfare as possible.
2 - If you do try going around the block, maybe trade off between the manual and power chairs, just so that she doesn't obsess on the power chair being the one that you're taking out.
3 - Perhaps you should put the manual chair next to her crate when you take the other one. For a while, at least.
There are other steps that I can think if, but those would probably be the start.
Also, aside from what she normally has in her crate, maybe you can hold off on the chew until you come back, as a kind of reinforcement that you returned, and all is well. Ending on a high note.
I know that both children and dogs begin making negative associations with treats if the get them when they are stressed. Most people, even my own mother, think/thought that giving a treat helps with something that you don't care for.
She used to give us a piece of Wrigley's Spearmint gum when we were headed to the dentist, which isn't my favorite place. To this day, I can't stand even the smell of that gum, or much of anything spearmint. I'm fine with peppermint, etc. I know that it's from that gum, but I can't change the dislike that I have for it.
Now, probably, we are overthinking a single experience, but it's good to try to avoid any pitfalls, since you are thinking of it.
We are having to do the same with the baby, since she was born 2 weeks before our first lockdown, and really didn't get to go anywhere. And, when she did, everyone was masked.
I just said in another post that the neighbor, very nice guy, would say 'Hi,' to her from across their yards, and she'd cry.
These days they are slowly letting her meet other people (still distanced for most.), she's able to go to the park, a horse riding facility next to somewhere her mom goes, and Home Depot once. She's getting better, and I can see that, when it's safe to do so, that she'll be a social butterfly.
In the same way, Arwen's world needs to expand, but in a non-threatening, non-scary way. What you and I consider not-threatening and non-scary will be different from her thoughts on the matter, I'm sure.
I know how much work a puppy is, but you've been wanting to go back to basics with Arwen anyway. Maybe she can go for walks to let her walk on her walking skills, etc. as well as sit, down, and, of course, housebreaking basics.
Eventually, you may want to include her in some of your trips out in the van.
I touched on toys in the crate, but there is one more toy issue that can develop.
If a toy is always in her crate, that's one thing, but I would be careful not to give her a toy just for when you go out just yet. Once she relaxes and is comfortable with you going out in the van with your power chair, then she might love it.
But, when you give a dog a particular toy before a stressful situation, it can become a trigger, like a flavor of gum.
I've seen dogs who see a toy only given to them before their people go out, or they go to the vet, etc., and the getting the toy becomes a trigger for their anxiety, because they know that something that they don't like is coming.
Good luck, and keep us posted.
(I still haven't been in my mail. And, now I'm heading to heat some food. I'll check right now, so that I can let you know if it came.)
Love All Life, Thank You For Posting! :)