I can't comment re #1, but for the others:
2: There are two ways to get the 5.x versions of the official holds:
a) Steam: TSS and GatEB are standalone games, the older holds are only available as DLC for GatEB
b) Caravel: If you have an older game registered in your account *and* an active CaravelNet subscription, downloading the official holds within TSS will get you a 5.0ised version of the hold with the challenges etc.
Note that the Steam version of the GatEB hold in particular differs slightly from the Caravel version -- not a lot, but there are a few minor changes to some rooms. The Steam version is the current one in terms of the high score spider.
3: I believe the free month of CaravelNet only comes with purchases direct from Caravel, not on Steam. But if you have CaravelNet, you can use the online features within the Steam version.
Purchases on Caravel should also give you Steam keys from
http://forum.caravelgames.com/member.php?Action=purchases .
4: They will import from the GOG and Caravel versions into Steam, but not currently back the other way around (the Steam engine version is currently 5.1.0.6545, while the Caravel TSS is 5.1.0.6512 -- you can only import in the same or higher version).
Some notes about merging that apply regardless of Caravel/GOG/Steam:
Merging is based on an internal ID, not the profile name. If historically you've used the import function to clone a single profile between all your GOG games before you started, they'll have the same ID and merge into a single profile in TSS or the Steam versions. If you've created a new player in each game, you have five different players that won't merge.
If you have different players, you can still merge progress by using the 'Export Saves Only' function from one player and importing into another. That allows you to bring all your saves into a single player record so you can restore to wherever. However, it will not merge recorded demos or ownership of your user-created holds.
