Before I give my own answer, I just want to say... don't put it all in Wellesley. I can think of several arguments to using that(active, no international, lifestrategy is more diversified/lower cost, etc.), but my primary reason for shooting it down is tax efficiency. If you tried really hard you can find something less tax efficient, but it won't be easy. It is active, regularly has short and long term gains distributions, and it is really heavy in taxable bonds. No, it is not tax efficient, and you don't want it in a taxable account. I also don't agree that it has low turnover. 31% turnover might be low compared to highly active funds, but by Vanguard standards that is really high. A lot of Vanguard active funds have turnover that is less than 10%. Even if you wanted a simple one fund solution to achieve a 40/60 you could accomplish the same thing with a lifestrategy fund. Again, I wouldn't do that because it isn't tax efficient.
1) Figuring out asset allocation: I appreciate that you are trying to help your mom, but I don't think this is a question you can answer by yourself. Part of the question is logical, what is the best portfolio for the goal? Part of the question is emotional, how does she feel about volatility? You can't answer the second part of that question for her. Here is how I would handle this.
Show her this:

Source:
https://personal.vanguard.com/us/insights/investingtruths/investing-truth-about-riskLook at the bottom number, the worst year. Ask her how she would feel if her portfolio did that in a crisis. I would start with the 60/40, but you know her better. If the -26.6% freaks her out, move to the left. Keep moving to the left until you find something you can both agree with.
2) Yes, try to put the stock index funds in the taxable, and bonds in the IRA. Making sure you get the AA right is priority number 1, but then focus on tax efficiency. Put all the stocks in the taxable, bonds in the IRA, but if you have to put bonds in the taxable to achieve AA then that is the priority and you will have a small amount of bonds in the taxable.
3) They are better in trad, but if you have to put it in the Roth as well. Here is the logic, the Roth never gets taxed again, so all else equal, you want the Roth growing faster than the Trad.
Tax efficiency, step by step: Put stock index funds in taxable, put bonds in pre-tax, if you need more bonds put them in Roth, if you still need bonds after that then put them in taxable, if you didn't put bonds in the Roth put your most aggressive investments(international or small cap) in the Roth, now fill in the blanks with stock index funds.