Salemance The concern about pinsir wasn't seismic toss (which is pretty awful at this point), it was the 55 BP normal type move coming off of 125 base attack. This combined with it's surprisingly decent bulk make it a bit too much for this point of the game.
Besides that though, looks like you thought all this through, so that's good. I certainly don't see any problem with delaying Timburr or Return until just after the forest.
As for Tangela, this is my main reason for concern...
0+ Atk Fletchling peck(35 BP because trollfreak) vs. 0 HP / 0 Def Tangela: 12-14 (24.4 - 28.5%) -- 22.8% chance to 4HKO
That's not even enough for breaking regenerator. At this point of the game, one could quite feasibly make a team containing just slowpoke and tangela and stall indefinitely. Slowpoke honestly should be ok, but of the mons available at this point, very few of them can even break regenerator:
venonat: doesn't even get leech life until level 17, let alone a respectable bug STAB
fletchling: see above calc
venipede: poison sting is so bad bidoof would probably have a better chance of breaking through.
larvesta: omg, a positive matchup! 0 SpA Larvesta ember vs. 0 HP / 0 SpD Tangela: 20-26 (40.8 - 53%) -- 9% chance to 2HKO
foonguss: level up movepool is hardwalled, but it hardwalls tangela back so I guess you can toxic stall it
Here's bunnelby for our base line: 0+ Atk Huge Power Bunnelby Tackle vs. 0 HP / 0 Def Tangela: 12-15 (24.4 - 30.6%) -- 99% chance to 4HKO
So yeah, it's either start charmander, grab larvesta, add toxic to your team, or get hardwalled by the first regenerator core we decide to throw at the player. At least slowpoke can get wrecked by the plethora of grass types that we have in the forest. For the record, tangela has a BST of 435, compared to slowpoke's BST of 315. Even at low-ish levels, those stat differences make a difference.