The IO is correct, under the law if you are returning to the previously registered accomodation within the check out period, you do not have to create a new tm30 record. However, if you need to interact with the imigration office during your stay some offices require an updated TM30 where your check in date is greater than your last arrival date. Get your girlfriend to set up an online account which means you can do it from home.
to register an address an ID card and blue book are required, (copies uploaded). This gives a user id (email address) & site password. Whoever has the ID and password can create records and print pdf. Its the owner/landlord/mamager/housemaster's accountability to ensure its done, but any authorised person can take responsibilty for actually doing the reporting if they have the id and password.
If your in a long term rental, try and arrange amicably that you get access to the online account, and do any required entries yourself. I'm assuming the agent has already created one rather than them visiting the IO everytime.
A lot of the Bangkok condos have line groups for owners (primarily Thai), and they unofficially agree the minimum rental rates and don't like to be found to be not playing ball.
Agents in Thailand have a vested interest in you taking the 2nd option as you will need there help at various stages. The first option requires no agency support and can be done by yourself end to end, although agents can be helpful negotiating through the required supporting paperwork requirements and translation issues.
but technically your prepared to owe the government upto 30% of it in tax if its paid into your account? While loans aren't taxable, it can only be classed as a loan if it has documentation detailing a reypayment plan and interest rate, Not saying you"ll pay it, just that technically its owed, and cant have tax credited off from your home country.