@BartonFink I'll be honest, I didn't read all the posts, just the first page, but it sounds like the problem is your tactics.
You have to think pretty far ahead in this game. You can only play one card per turn, but you have be setting stuff up about 3-4 turns ahead. Likewise, if you see your opponent setting something up, you have to gauge when the best time to interfere is. Often is not immediately.
You can tell inexperienced players from good players by the tactics they use, not by the decks they play. Two players with a poison deck could play very differently, because it's more so about the tactics. When to pass, when to push, what units to attack, what ones to ignore, what order to play your cards in according to what's already happened, or what interference your opponent ran. Your poison deck should be obliterating most everything. If it's not, you're not playing it right (or you created your own poison deck that might have a few hiccups in it, which I respect way more than using the meta).
Card advantage is huge in this game. It's not all about power. If a player played strategically, then yes, that 1 card advantage at the end should win them the game. That's why it's so vital to get card advantage. Also, if you play strategically, it's fairly easy to thwart that card advantage as the player who goes second to last. Maybe it's better to push for a victory on the second round because your opponent drew a crap second hand in a deck that runs tons of tactics/specials (meaning they won't have enough power to play on the board for that round). You have to think about these things. I have won from a 2 card deficit because of the exact scenario I described above. I had 4 units, they had 6 cards, about 4 of which were specials. By the time they put their units down, they did not have enough power to overcome my own. Boom, we didn't even go into the third round.
Everything you described on the first page sounds like poor decision making in tactics. You need to prepare for that last card. You can't just expect to win on a 15 point lead. There's a reason they haven't forfeited yet. Once you know it, you play the whole game around that, and then it no longer becomes a concern.