[mothered]

Every SE Is based on It's merit, hence It's somewhat difficult to be definitive In answering your questions.

Generally speaking:

Is there a general limit on the frequency of SE for Amazon returns (like a certain number of times per month) or a limit on a price per order?

Set yourself a (realistic) goal, stick by It and don't be greedy. High value Items tend to attract more attention, so careful planning and execution Is paramount.

Also, is SEing easier with items sold by Amazon, 3rd party, or 3rd party FBA?

As with every SE, you're more so exploiting the human firewall. Do your research, prepare and formulate your method and execute It accordingly.