I'm pretty new to this so maybe someone more knowledgeable can chime in here.
I know people have been successful, using Pokepatch, to patch their Pokemon Diamond and Pearl roms in order to read GBA Pokemon roms loaded into slot 2 carts, like the EZ Flash V 3in1 expansion cart. Reason is they have to patch is that Pokemon Diamond/Pearl and your Platinum games, expects to read the game save info from the GBA cart from flash memory. The EZ Flash 3in1 cart, which holds GBA roms, uses SRAM memory for its game saves, so people have to patch the D/P (Diamond/Pearl) roms to have it look in SRAM instead.
Your situation is different obviously, since you are using a real Pokemon DS cart and you can't patch your Pokemon Platinum cart to have it look for GBA saves in SRAM, not Flash with appropriate slot 2 cart. I don't believe there exist a slot 2 gba flash cart that emulates the weird flash memory type of the GBA pokemon games.
Furthermore, even if one existed, a lot of GBA slot 2 carts for DS are out of stock or hard to get. The easiest to get, and most recommended here based on availability, is the EZ Flash V 3in1 expansion cart. Since it uses SRAM to store it saves you can't use your retail Pokemon DS cart.
You'd be like everyone else and have to use a slot 1 cart that has EZ Flash V 3in1 support (ie. Ez Flash V, Acekard2/2i, M3/M3iZero (with patch), CycloDS, etc), patch the Pokemon D/P/P rom with Pokepatch. Install the pokepatched Pokemon rom on your microSD card inside your slot 1 card, and then the Pokemon D/P/P rom should be able to read the game save from the GBA rom on your EZ Flash 3in1 slot 2 car( a clean GBA pokemon ROM, and I think SRAM patched via GTATA, that is flashed to NOR, not PSRAM in the 3in1). I'm assuming Pokepatch (current version is v4) supports Platinum. It was original made however for Pearl and Diamond so I don't know.
Oh yeah, you'd have to also get your save from your retail Pokemon DS cart onto your slot 1 flash cart. There is a guide on dumping DS roms, and I think it also meant it would dump your save.
Here it is. It's for mac, but a pc guide is link somewhere in that same thread. You basically use wifi from your DS and dump the rom (and its save) from your retail cart onto your computer running a FTP server.
Pokemon games are just weird. Normal GBA linkage, from what I understand, doesn't require all these hoops. Pokemon DS games interact with gba saves so that's what causes the hassles.
So short answer to your first question, is no. You can't use the retail Pokemon DS cart to migrate from a slot 2 gba flash cart.
Your second question, there is a slot 2 GBA cart that has RTC. I think it's called the M3 Perfect lite. It's hard to find now and it's expensive, like $70 or more. Most people these days, without RTC slot2 gba carts, use the "clock fix" option in the GTATA tool and use it to patch their GBA pokemon rom. I haven't tried it, but it's a way to get these pesky pokemon without RTC flash cart.
This
thread might explain things better, like why you can't use your retail Pokemon DS cart.
Even more reading if you are ready to tackle migrating pokemon, here's the
epic thread on using EZ V Flash 3in1 and migrating pokemon (if using a slot 1 card besides EZ Flash V, instead like the Acekard, R4, etc). it's got a link to Pokepatch v4 on page 8, although i didn't test the download link. You probably can find it somewhere. Maybe there's a newer one now. Don't know. If you've got a EZ Flash V slot 1 card, things are probably easier to use with 3in1 since they are both made by the same company. But it seems other slot 1 cards are more popular and they offer various support for the 3in1 or even the new 3in1+, so that's why people use non-EZ Flash V slot 1 carts with their EZ Flash V 3in1 (or 3in1+).
Anyway, probably way more info than you wanted to know. I just researched all these stuff on a different issue (trying to find a tool to erase corrupted SRAM on my 3in1 cart, I did, the GBALdr tool), so I had this info fresh in my mind.
It's a lot of steps to be able to migrate pokemon from gba flashcart slot 2 to slot 1 flashcart. You have the added step of needing to dump your game save from your retail game cart onto your pc then to your slot1 card's microSD as well.
If you got the money, it would be far easier just to buy the GBA pokemon carts for migration and keep using your Pokemon Platinum retail cart. Heh. It's possible to use your current Pokemon Platinum save data taken from your retail cart, but you'd have to buy all the stuff (slot 1 card, Ez V flash 3in1) and download all those tools, patch the DS and GBA roms, transfer your game save off your retail cart, and troubleshoot any problems.
