I believe that neither the AGI nor the MAGI are affected by your standard or itemized deductions, exemptions, etc. The only thing that affects them is the above-the-line deductions (literally, above the line on the 1040 that says "This is your Adjusted Gross Income"). That line gives you your AGI. Then to calculate your Modified Adjusted Gross Income (MAGI), you have to add back in a few of those deductions that are not allowed to be counted for that. This doesn't get done on the 1040 itself, as far as I can see.
In my simple case, my gross income will be 72k this year, and I'm single and have a 401(k) with my employer. To deduct a traditional IRA, I need to get my MAGI below 60k. I put about 4k of Roth contributions into my 401(k) already, and that doesn't help at all. But I'll be maxing out the rest as traditional 401(k), so that will reduce my income line by 13.5k to 58.5k. My HSA contribution of 3.3k and the 5.5k traditional IRA itself are above-the-line deductions, and will then reduce my AGI to 49.7k. To calculate my MAGI, I have to add the IRA back in, though (you can't use the IRA itself to make yourself eligible for an IRA), so that makes my MAGI 55.2k. Still well under the 60k. At least I think that's right - this is my first time doing this, and when I saw yours were so different I thought maybe I was doing it wrong.
As to your 401(k) contributions, if they allow 30% of each type then I think you are right, that will help you max it. But it's odd that they force you to split it that way. I know the plan rules can be arbitrary - my first 401(k) had a 25% limit and no Roth option. My current has a 75% limit - which I'm currently using because I had it set so low in the first half of the year. My last semi monthly paycheck was only $350. :)
Probably the reason is that so few employees really try to hit the yearly limit. If you're just putting 15% away, a 30% limit doesn't bother you. So I guess it's a Mustachian problem. :)