Sideloaded and Downloaded tpks are the same thing. Its pretty much describing where the tpks are coming from. You could take a tpk you downloaded in the Collector app and sideload it onto another device and there are no credits be used in this workflow. You could create a tpk in desktop and sideload that to app as well and this as well wont burn through any credits.
Where credits would come into play is if you took that tpk you generated in desktop and uploaded it to AGOL and published a tiled service. That would cost some storage and tile generation credits but from that hosted tiled service you could download tpks from if you enable the tiled service for offline use.
Currently the only way to limit the max extent in Collector is through the basemaps extent. The app has a bookmark displayed by default that is the maps default extent and could be used to help return the field user to the default extent of the map if they pan out of view.