Ok... you got a few different questions...
First, on the tarp thing... that only really will work if you're leaving htem in there for very short time periods..... A parked vehicle will get hot in the sun no matter what you do to it... picture a cargo van.. no windows but on the 2 doors... leave it in the sun for 30 minutes and then open the back loading door... inferno.. there are a lot of other factors in heat gain with vehicles thatn just the glass... BUT.. the tarps will SLOW the heat gain (but not STOP)
Question 2: How dark you get tint is generally the consumers call, you go as dark as you feel you will be comfortable trying to look through (and will still be legal as well). If you'r wondering how dark to go on a sunroof... that's the wrong approach (but o.k. ~ only tinter's realize it

)
~~~window films do not block more or less heat according to their darkness. Smoked 5% (limo) tint will not block any more heat than a light 35% film... what you need to look for if heat is your issue is actual construction of the film and not darkness. The best heat rejecting film are those with a heavy metalized composition ~ like films with a silvery/reflective appearance (not the black/smoke colored films)
#3 ~ Silver reflective film.. the film I know you are asking about does exist, it's called R20 and R35 (R20 is darker). R-series films are architectural films and are not designed to be applied to cars ~ and are also illegal for that use in all 50 states. If your state allows any reflective type of film at all ~ it would be a maximum of 20% visible light relectance. (some states allow no reflectance)
The R20 film is the stuff that looks like a bathroom mirror.. like chrome plated glass. It takes an VERY VERY long time to dry and does not stick very well to auto glass due to that it's adhesive is of a different type than that used in auto films. It also scratches easily since it does not have the good scratch resistant coating that automotive films have (house film don't need it, no one rolls home windows up and down 10 times a day into a gritty sandy window track!)
~~ but yes, you do see the film around on occasion ~ some people insist on it and some tinter's risk the HUGE fine and install it.
If you are interested in heat reduction I would suggest going with a SEMI-reflective film (such as Johnson ES series, Llumar Platinum Plus or Madico Kool Chrome). Those films will adhere well to the glass, last a very long time and reject a good amount of heat... but again... remember they only SLOW the heat gain, not stop it (if parked in the sun).
#4 ~~One thing about E's that you should ask a prospective tinter... the E door panels NEED to be popped off!!!! The window's dust seal has TWO dust seals to it (you see one), but the film if not tucked past the SECOND seal will eventually peel (not good). So, the rememdy is to pull the panel off ~ no big deal, it's just a few screws here and there.
also... ask the tinter if he's ever taken an E door apart before. Important... if it's the tinters first time at it, he'll break the the first door panel he does (on an E) ~ even I did it!! On the latch edge of the door, you see in the bottom corner of the window there is a triangular peice of plastic trim ~ that thing takes a certain move/gentle finesse to pop off... easy after the 1st time! It's just a weird part.... remind him that it breaks easy if not attentive and you should be ok (and if he breaks it... it's an easy fix with some super glue :shock: )
hope that helped