It is a bit tricky yeah but the basic ways I'd describe them would be:
Seed - Is the value that generates a lot of things, Trainer and Secret ID, encounters, stats, shininess and even stuff like NPC positions in some cases. For example, the Seed to get the Umbreon/Espeon I gave you is 8228F218
Frame - Is When you get the things generated from the Seed. For example, your Umbreon stats were generated on Frame 4, and Espeon's were on Frame 11.
I needed to change the TID/SID combo because that also needed to match in order for the Umbreon/Espeon to be marked as fully legal.
Now for the tools to use, RNG reporter is good, though I've mostly used PokeFinder since I found that to be better/easier for me to use. And in this specific case, I use a script to check Colosseum's starters.
Lastly, to be honest IVs don't really matter or come into play a lot if you're doing a casual playthrough, they're more prevalent on a Competitive setting so it's not like the game is unbeatable if you don't get max IVs.