As an analyst, I like the idea of having an LP loss reduction when there's someone afk under certain criteria (e.g. afker must not be premade with you (obviously), afker must have been afk since minute X (10 mins? 15 mins? 20mins?), afker must remain afk for the rest of the match). These strict rules will make the reduction apply in a limited amount of unfair matches, but it will strongly reduce the amount of abuse that can be caused. The only downside I still feel is that people might (uncertain) be stimulated to say bad things like "plz go afk you lost this game!" While this will obviously not cause these people to go afk, its still negative behaviour sippling into the game.
Now, as I said, I like having LP loss reduction as a compensation, however, not a loss prevented. Why is this? Loss prevented also forgives you the MMR loss. As you know, the amount of AFKers in both teams evens out in the long run. Forgiving MMR would cause an inflation of MMR which would require harder MMR resets at the end of the seasons. Also, getting higher rank would get significantly easier by the end of the season unless the tier system adjusts for the total amount of MMR.
LP, however, is kind of like a short term currency. In the long term, LP is worthless. It doesn't matter if you start bronze V or plat 1 with the same MMR, you will still be at the same tier after 500 games. However... In the short time, LP plays a great role where it can determine whether make you barely reach a new tier or not. This can play a great deal at the end of the season, or when you play your last matches for that season. And since the occurence of an AFKer has great impact in the short term, but less in the long term, it would make sense to have the impact of this reduced on the currency that is important for the short term.
Whether or not it should also prevent losses for series, I'm not sure. Series is in fact a short term thing, but this might be important enough for people to search for ways to abuse it. Even with all the protections against abuse there are, it will never be 100% waterproof.