@TheSlayerofSloppyJoes To answer some of your questions:
1) From what I understand, Blast provides the listed attack bonus for each field closer to the target than its maximum range. For example, the first Blast unit we have access to is the Howitzer. It's also the only one which can be used in battles not subject to the 1.5 scale battlefield. It has a range of 6 and a Blast bonus (coincidentally) of 6. I'm unsure of whether the bonus is applied even at max range, but what I do know is that for each hex closer to the target beyond max range, the Howitzer will gain +6 attack for a maximum bonus of either +30 or +36 attack added to its attack value when directly adjacent to a unit. In 1.5 scale battles (Progressive and higher units present on the field), the bonus is still applied in the same way but condensed in the same way the range is. A Medusa's 50 range ultimately ends up being only 34 in battle, but if directly adjacent to a unit it's still going to receive the full +294 or +300 attack bonus for a range of 50. For what it's worth, I'm also unsure of whether the Blast bonus is applied before or after military bonuses, but I'm inclined to believe that it's applied after (like terrain and unit bonuses are). However, I feel like if that were the case then this whole thing with the Medusa might not be possible so perhaps Blast's bonus *is* affected by military boost?
Incidentally, the Plasma Artillery, at least for me, does have a Blast value of 3 displayed in-game.
2) The Medusa was given such a range value essentially to mean 'infinite', so that it's always able to hit any (non-flying) unit regardless of battlefield positioning. Unfortunately that's led to some unforseen problems due to that range's interaction with Blast, as outlined above and in previous posts.
3) To be fair, I think the unit descriptions on the wiki are for what's applicable in the early ages rather than the game as a whole, although artillery remain the weakest units in each age (from an attack power standpoint) until FE with the Rail Gun. Probably because that's also around the point where each unit's combined movement and range is roughly the same, and the differences end up coming down to abilities and type matchups rather than stats.