I don't have access to Jepp charts, but I do have access to SkyVector and this March 2012 Jepp legend.
1. On a Jeppesen IFR low enroute chart, there's occasionally a different altitude below/next to the MEA with a "T" next to it. (For example: V16 between GACEB and SHAKY, there's a "2900T" next to the 4500 MEA.) What is this "T" altitude?
From page ENROUTE-6 on the Jepp legend:
35— Minimum Obstruction Clearance Altitude (MOCA).
This corresponds with the Government chart which has, on V16 between GACEB and SHAKY, the MEA
4500 above the MOCA
2. Similar to the question above, but on a Jeppesen high enroute chart, there's occasionally a "D" below/next to a given altitude. (For example: Q20 between FUSCO and JCT, there's 18000, with "24000D" below it.) What is this "D" altitude?
The 2012 Jepp legend does not list what a "D" suffix means on an altitude, but it does say (referring to the above screengrab)
34— GPS MEA.
We can compare to the Government legend:
and we can say with confidence that the "D" suffix lists the MEA for a DME/DME/IRU-equipped aircraft, just as "G" indicates the MEA for a GPS-equipped aircraft.
3. On a Jeppesen STAR, there are a combination of dashed and solid course lines. (For example: On KDFW's Sasie 6 Arrival, there are several dashed course lines prior to Sasie, after which the course is marked by a solid line.) What is the difference between the dashed and solid lines?
From page SID/STAR-4 of the Jepp legend, a thick solid line represents a "common course used by multiple transitions" while a thick dashed line represents a "transition track." This is analogous to the thick black line for common routes and thin black line for transition routes on Government charts.