Summoner, eidolon, animal companion and pack flanking

[Level 20 NPC]

Hello everyone, if a summoner has an animal companion (thanks to a dip or feats) he can qualify for pack flanking.
If he rides his eidolon using the horsemaster's saddle then his eidolon has access to pack flanking.
Also, eidolons are indeed companion creatures, so...
If my druid 1/summoner X rode his eidolon, could he use pack flanking with the eidolon?

Of course this is not RAI but are there any FAQs/rules i'm not aware of or is it RAW?

[Mark Hoover 330]

By RAW the only way the Eidolon gets Pack Flanking is if it takes the TW feat or if the PC shares it to the Eidolon, like in the example of the magic saddle.

[Valandil Ancalime]

A dip in cavalier gets you Tactician.

Is there another way to gain Tactician?
Does Mount allow you to take Pack Flanking?
Is there a Cavalier archtype that gives an Animal Companion instead of a mount?
Would sum4(+ feats nature soul/animal ally)/cav1/sumX work? Animal Ally says, "If you later gain an animal companion through another source (such as the Animal domain, divine bond, hunter’s bond, mount, or nature bond class features), the effective druid level granted by this feat stacks with that granted by other sources."