1. Yes you can run the batteries in series. Two 2s in series = 4s. The esc is 4s rated and a quick search reveals some people using this esc in Summits on 4s. Also Ohm's law actually helps the situation. A given combination (gearing, weight, terrain) will draw a certain amount of watts. Ohm's law is watts = voltage multiplied by amps. So let's say you're drawing 50amps on 7.4v (2s) which equals 370 watts. Now take 370 watts and divide by 14.8, you get 25amps. Less amps means things run cooler. Without an amp meter there's no way to tell how many amps you're actually drawing but ohm's law is in effect regardless. The caveat here is the load must remain the same in order to draw the same wattage at 2s and 4s. We keep the load the same by keeping top speed the same which requires adjusting gearing. Sorry if that's a bunch of information just trying to illustrate some principles and show why 4s is better for all situations. The only advantage to parallel is the longer run time but even that is negated at least somewhat by the increased amp draw.

2. Gearing. You have Traxxas Titans and I'd go by Traxxas' recommendations. The 3906 manual lists 18/66 as the stock gearing and that they don't recommend going higher on the pinion or lower on the spur than that. Personally I'd go with 18/70 just to be a little safer and since you have those gears already. Monitor temps in 5min intervals and go from there. Remember you have 2 speeds in that truck which can reduce temps if you use low gear through stuff that creates a high load, tall grass, mud, when you're kind of stuck, etc.
Here's the manual for the truck https://traxxas.com/sites/default/fi...019-020823.pdf

3. Upgrades. It's very easy for upgrades to snowball on a Maxx truck and before you know it you have tons of $ invested. This is due in part to the truck needing a bunch of upgrades and so many upgrades being available. However if you stay brushed you're less likely to "need" upgrades unless you ram it into stuff constantly. The 3906 is also the least durable. I recommend only upgrading the steering with the steering bellcrank from a newer truck and use the black spring. There aren't any servo mounted servo savers that can handle turning tires that size, they simply flex too much. The newer Traxxas bellcrank has the servo saver built in and is tighter resulting in more positive steering. Maybe upgrading the diffs to the newer ones if you break the current ones.

Anything beyond that is not worth investing in the 3906 platform in my opinion. The trans can't handle brushless without upgrades that are long discontinued, the axles and driveshafts won't handle it either. Cvd axles aren't available for the old style knuckles of that truck requiring those to be upgraded too. See the snowball effect? If you want brushless get a newer EMaxx or better yet something other than an EMaxx and leave your 3906 mostly stock.