Otto's is Irresistable Dance. The PHB name, anyway.

Counterspelling in general is bad unless you are hyper-specialized in it. Since you aren't, its not worth the effort. You are better off readying an action to do something else that'll disrupt the spellcasting. Solid Fog blocks Line of Sight, which kills more targeted spells. Otherwise, an Archmaged Reached Irresistable Dance shuts a spellcaster down just as fast midcast. Something else that would work would be just about any damage spell. Even if it only does ~35 damage, thats a DC50 +spell level concentration check. Scorching Ray can handle that, and its only a 2nd level spell!

Arcane Thesis does 2 things. When you select it, you pick a spell. Your caster level for that spell is increased by 2. When you apply metamagic to that spell, the level adjustment is reduced by 1. The sage ruled that the total adjusted level of the spell can't be less than the origional level of the spell (so no Energy Subbed Fireballs as 2nd level spells), but for Enervation, its amazing. You will want to Empower your Enervations, and with AT, your Empower only increases it to a 5th level spell, instead of the usual 6th. Sometimes you'll want to Quicken your Enervations. AT reduces Quicken to a +3 for Enervation, so a Quickened Enervation is a 7th level spell, instead of 8th. And its effects stack. So a Quickened Empowered Enervation is (4 + 3 + 1)=8th level spell, instead of 10th (uncastable). Its really good if you plan on using one spell a lot, and Enervation is pretty universally accepted as one of the best core spells to take AT for.

As for your last question, take Archmage again and get another SLA. Its better than True Lore. WAY better. SLA effectively turns one of your 5th level slots into a 8th or 9th level slots. Thats a great trade up!