Does HELOC deduction assume that oldest HELOC expenses are paid off first, when making payments on a HELOC that is a mix of home improvements and personal expenses?

The deductible part of any mortgage loan (no matter the technical form -- conventional mortgage, line of credit, fixed equity loan) is your acquisition debt.  Acquisition debt is debt acquired to buy, build, or substantially improve your home.  For example, if you have a conventional mortgage used to buy the home, that is now partially paid off, and a separate line of credit, only the conventional mortgage is acquisition debt unless you use the line of credit for substantial improvements.  If so, you can deduct part or all of the HELOC debt depending on how you spent it.  

You can consider that you are paying off non-deductible debt first.  For example, if you open an HELOC and withdraw $10,000 to remodel the kitchen and $5000 for a vacation, and you have since paid the HELOC down to $11,000 balance, you can still consider you have $10,000 of acquisition debt.  However, if you paid the loan off to $8,000, and then took another $5,000 vacation, you can't bump the acquisition debt back up to $10,000.  You can only consider HELOC draws as paying for improvements if you make the draw within 90 days from the completion of the work.    After 90 days, it is non-deductible personal debt, even if you are making (very late) payments to the contractor.

You can either figure your deductible interest month to month, if you keep the records, or you can use a first month-last month average. For example, in January, your HELOC balance is $11,000, of which $10,000 is for an improvement.  In June you borrow $5,000 more for another improvement ($16,000 with $15,000 being acquisition debt), and by December your loan balance is $14,000, all of which is acquisition debt.  For January, 91.11% was acquisition debt, and in December, 100% was acquisition debt, then for the whole year you can deduct 95.55% of the interest.

Be aware of the time frame.  You can only count as acquisition debt, money you withdrew to pay for substatial improvements before, during, and up to 90 days after the completion of the work.  Past 90 days, and it can't be acquisition debt.

Also be aware that, for determining cost basis, an improvement is any permanent upgrade that adds value to the home or increases its useful lifespan, but for the deduction of acquisition debt interest, it must be a substantial improvement.  There is no strong guidance on the difference between improvement and substantial improvement that I can find, although in the back of my mind, I think I read once that "substantial" is more than 10% of the value of the home.  The concern is you said "expenses" which could be almost anything.

Repairs keep the property in as-is or as-was condition and don't add value or extend the useful life, so a repair is not any kind of improvement much less a "substantial" improvement.  A new roof would be a substantial improvement.  Planting a tree adds value to the property and would be considered a capital improvement, but it probably isn't substantial.  Be careful of what you claim as a "substantial improvement" for purposes of determining your deductible acquisition debt interest, and keep records as long as you own the home plus at least 3 years after you sell.