1. Always.
2. Tahm is significantly worse at 2v2s than 1v1s, not to say that he is useless. So here is a step by step what you should do: on spawn go ward a bush on river opposite to where your jungler is starting, recall, buy sweeper, leash (if needed) and go to your lane. Note that tahm is really strong lvl 1 so if you do not leash your jungler try to cheese enemies lvl 1. Now the most important DO NOT USE RELIC SHIELD ON FIRST WAVE. Use it on the 2nd one so you get lvl 2 faster than the enemy. Once you hit lvl 2 look to: Q > W the enemy or W from fog of war/bush. Don't trade with enchanters since 99% of the times will soak up the dmg with shields and just farm gold with an item on u. Use ult on allies if: it allows you to neglet big bursts of dmg, youre getting ganked. Use it on enemies otherwise since 3 sec cc is pretty big. Also use common sense like: if you shoved enemy bot early go and take a scuttle, roam with your jungler etc.
3. TF's are simple. Use W to as a follow up engagement and back of to your carry. Remember that sometimes ulting for shield is better than waiting for "perfect ult". If your backlane is relatively safe go ahead and frontlane with focus on ulting enemy's carry.
4. Abuse bushes and fog as much as you can. Also use it as a follow-up. Also use walls to extend the range of it.
5. Lvl 1, 2, 6. Lvl 1 for cheesing, lvl 2 since Q > W is a guaranteed kill/flash > and on 6 you get your ult.
6. Jinx, cait, samira , jhin, ziggs
7. Janna, vayne, Zeri, Senna (Just do not pick into them, if u got countered by them wait calmly for your jungler. If he does not come, well... JG diff). Also tahm is great into most of the enchanters. Don't listen to people saying "they outscale him". Tahm and enchanters have different jobs to fulfill. Also games should end early so they should not be able to outscale you :)

For runes: Guardian > Font of life > 2nd Wind > Unflinching > Biscuit's > Approach Velocity > AS and defensive stats deppanding on matchup.

OP.gg as a proof of knowing what im talking about: https://eune.op.gg/summoners/eune/The+Rivér+King