These are all my personal opinions, feel free to rebut them. This is coming from an artist who has personally tweeked their TOS mid-commission.

Simple edits here and there are fine; it's when the TOS is changed drastically that you should be wary. I'd like to compare that to when websites change their TOS. Okay, it's a little bit different, but it's essentially the same idea: They change their policy, newcomers as well as current consumers read the terms, and whether or not they choose to be subject to those terms is up to them. In a case like this, the client(s) will have to sign to the new terms before the product can be delivered.

Your second and third questions can be answered with one word: logs.

As an artist, if you ever update your TOS, even if it's the smallest detail, let your clients know and date the edit. This not only serves as proof of an updated TOS, it also serves as proof that a client (or numerous) have signed a previous version of these terms and should review the newest edit before proceeding with their commission. As a client, you should keep a copy of the TOS you signed, including the date you agreed to it/it was written. It can be a screencap, it can be an archived webpage, it can be copy-pasted into a word processor, it can be whatever, so long as you have a dated copy. That way, if ever there is an update to the TOS, you can not only compare the two but also have proof of the terms you agreed to.

Hope this helps!

Absolutely not. The TOS when the commission is agreed to should remain in place until the commission is complete just like any other non-ongoing service contract that is signed. To do otherwise completely negates the point of a TOS because the artist can do anything they want, anytime they want and the customer has no way to know what is expected of them or what to expect when going in. That's a terrible way to do business.

A commission is not an ongoing service. Finish the commission as you agreed to it, which includes the TOS as it stood when the contract was signed.

A commissioner I now keep a record. Save the webpage or copy-paste into a doc I haven't settled on a method yet. I do this because I learned the hard way that artists as a group can not be trusted. That you would even consider imposing a new TOS on an existing contract only reinforces this belief.

As an artist you should not be changing your TOS so often it requires much effort. Note the date of the change, what it was and keep track of when the commission was agreed to. You should already do the third to know how long the commission has been sitting in your pending pile. I'm not the only commissioner that's been burned enough times to never allow a commission to exceed PayPal's dispute window.

I will note that the above is not about fixing spelling or similar changes that do not affect the intent of the TOS's contents.

- Yes and no. It depends what part of the TOS we're talking about. Things such as changes in refund policies should not be forced upon people who have already agreed for a different version of the TOS.
But copyright policies can be changed, as the artist still maintain the original copyright for the piece. Suddenly changing it to "You can't upload it anywhere" is of course a dickmove, but since websites change TOS, it may affect the artist as well. If for example FurAffinity suddenly change their TOS to allow them to profit off of people's art, then I think it's perfectly fair for an artist to enforce it if they change their TOS to not allowing anyone to upload their art to FurAffinity and apply this to past customers (By respectfully messaging them, not by accusing them of break of TOS by not instantly deleting the piece off the site without even knowing the artist changed their TOS)

- I always include the relevant points of my TOS (Such as stuff about refunds) directly in the PayPal invoice to ensure that the client is agreeing and signing the agreement when they pay the invoice.

- If it's not included on the PayPal invoice then it's to my understanding not technically signed, though it's very much accepted in the art community that simply giving a link to the TOS is enough, and purposefully breaking a TOS just because you didn't legally sign a contract is an asshole move.
But this pretty much means that unless the TOS was included in a PayPal invoice, the artist isn't legally protected and only PayPal's own TOS is taken into consideration (a.k.a. No finished product = Money back. So it's very important to include if you have any refund policies in case of cancelation)

1. No, however, if there is something in the course of the commission that makes an artist go 'you know what I can't do this/don't want to do this' that should be brought up to the client and be resolved, either by change or by cancelling.

2. Using things like Google Docs, it'll give you a revision history that can be referenced so you can check out the last changed date in correlation to the time the commission was taken.. Making sure to provide reference to the TOS or the whole TOS itself in the email to the client if using any invoices, helps with the clarity of that on the client's end.

3. If it wasn't sent, keep receipts of when the commission was paid for and keep your own copy of what the TOS was at the time of the commission. It would require foresight of course, but as I said before, if they kept the TOS on google docs, the revision history would be right there.

Easy- I email them a copy of my ToS at the time. In email attachment form it's uneditable and we can both reference it as needed. And no, an artist can't suddenly change the terms of the agreement.