Yes, inventories will still occur, but no transfer to the CAP will take place until a connection is present and the speed is 40K bps or greater. (I skipped over the "this value can be any value less than 40,000" line, so the 60K setting won't work. Sorry about that. Well, at least that means that only 56K modems on a good connections will get through.)
Smsls.bat is where the setting is made from my experience, but you may be able to do it elsewhere. The client doesn't really need smsls.bat once it is installed, but it doesn't hurt to leave it and it may be helpful in the future, such as if new clients are added or existing clients break in some way.
The slownet in smsls.bat relates to the sms client communication in general, not specifically pushing inventory. I not familiar with how the slownet setting gets propagated to the client (through the domain \smslogon\config\*.tcf and configured through SMS admin?).
The SMS client is a semi-independent entity once loaded. It checks its CAP for changes periodically and adjusts or fixes itself accordingly. Slownet is run on the client and tells the client whether the network connection (if any) is fast enough to transfer collected data. The small file copies over a 56K modem connection are generally fast enough that the user does not notice. Loading a new client over a 56K dial up, or copying full inventories, or doing a software distribution might cause some issues, though.
(I'm getting a step away from my experience, though; second opinions are welcome.)