It's tax time and my employer has royally screwed up. What can I do?

I am hoping someone could help me understand my options here as I'm not very well versed with taxes and such. I've always gotten a refund on my taxes and this is the first time I haven't. I went to do my income taxes last week and I'm in shock, apparently I owe the government roughly $1000 due to an error my employer made. Conveniently, the payroll lady has been on vacation and I have been unable to contact her about this. I've been holding off on calling the CRA (and dealing with those damn hold times) until I know more but I really don't like owing money and I'm getting impatient.

Here's the jist of it:

I made roughly 57k last year (I live in ON). My employer usually deducts 18% tax, even though I'm now realizing that puts me in the 20.5% bracket. (I'm not sure if it's their responsibility to adjust that based on my pay rate, this is the highest paying job I've ever had.) I'm assuming I did my taxes correctly, I just have one T4 and a T4A (I think that's what it is?) and nothing else to claim so it's pretty simple. No house, investments, retirement etc.

In September I went on maternity leave which ended around Christmas 2021. No issues with the mat leave pay through the government, they taxed my leave payments the proper way. My employer does a pay "top up" for 16 weeks that is supposed to be the same amount I would have made minus the mat leave payments from the government. Here's where the issue is. They only taxed those "top up" payments at 4.5%. Like the amount of tax they took off is less than my CPP contribution! I'm so confused about this.

When I returned to work they also claim that they overpaid me in September (roughly $700), and in November took a "loan" for that amount out on my employee account. Apparently their head office, not our local payroll lady, is responsible for figuring out LOA paychecks, and she said she thinks head office actually underpaid me and she would look into it. This was back in January, I still haven't heard back about that and they're not enforcing repayment for that yet.

Any advice? What kind of options do I have? I understand that if money was not mine to have I have to pay it back, my frustration is that I was told "don't worry, we do this all the time and we'll take care of it. No, you don't need to sign anything." And during my leave when I checked that they were paying me the correct amount: "Whatever head office pays you is your money, don't worry about it. They know what they're doing". Well apparently not!!! I live paycheck to paycheck so I can't just send the government a check for $1000 and if I understand correctly I have to pay them by the end of April. Is there any responsibility on my employer for this? I thought everything had to be taxed at 18% minimum because all my part-time jobs as a teen were, I just got all tax taken back because I only made like 10k a year then. And can I just tell the government to take my benefits/credits? Like the quarterly payments we get? Ugh, what a mess.