For the decimal issue, are you opening up the CSV in a text editor or excel to confirm? Filemaker treats number fields as raw numbers, and it wouldn't arbitrarily change 256.9 to 256.89999999995. I do know that excel will mask numbers it thinks are currency (any decimal really) to two decimal places sometimes (CSV or XLS/XLSX), but formatting all columns as text will show the complete decimal data.
As for everything else, sounds like the trials and tribulations of dealing with not so great data coming from quickbooks. If quickbooks is not escaping text string quotes in the CSV output, then either a different format that supports those strings (IE XLS) should be what's used for the cleanest data.
My recommendation would be to create a standalone file for "processing" your data, and script the first time through all the replaces you need to do for cleanup. You can then run data through this processing file to clean it up in the future, and automate the entire process.
It might also be worth it to get some advice from some of the companies dealing with Quickbooks and filemaker. Such as Productive Computing and Seedcode.
A search for QIF to CSV software will find desktop aps from $16 - $99.
Not to be redundant but you could avoid all these issues, and the time invovled in figuring out how to fix them by using FMBooks...:-)
That one is just odd. I started with a .csv out of QB. I open that with a text editor (TextWrangler) and can see that it is 256.90. I open that in excel, and it truncates it to 256.9, like I would expect Excel to treat something that it sees as a number. It's only when I import the .xlsx into FMP that it changes. Obviously, I can round it, or just display it with 2 decimal places in FMP. It's not actually a big deal… just unexpected behavior.