You should have no problem using Schedule 80 PVC conduit. It's easy to work with, and you can get pre-formed 90's for any corners (so you won't need a pipe bender).
You might be able to get away with Schedule 40, but since it will be exposed (and in a work area where it could get smashed with a wayward piece of lumber) I would go with Schedule 80.
Just make sure you do a better job than the previous "electrician", and use proper PVC conduit straps to attach the conduit to the wall / ceiling.
KeithS brings up a fair point. Make sure the first receptacle on the line is a GFCI receptacle, then feed the rest of the receptacles from the load side of the GFCI.
Liquidtight
If by "LT" you mean Liquidtight flexible conduit (metallic or nonmetallic), and you're looking for protection from physical damage. That's not an option, since it's not permitted to be used where subject to physical damage.
PVC
You'll have to use schedule 80 PVC, to provide protection from physical damage.
Calculate Fill
To calculate conduit fill for cables, you treat each cable as a single conductor and calculate the fill based on the major diameter of the cable (looked up from manufacturer documentation). For Southwire® Romex® SIMpull® 12/2 with ground cable, the major diameter is listed at 410 mils.
A = πr²
A = π * ((410/2)/1000)²
A = π * (0.205)²
A = π * 0.042025
A = 0.13202543126711106084639258818232 in.²
Since you're using the conduit as a sleeve, and not a complete conduit system, you should be able to fill the conduit to 100%. However, filling to 100% will make it impossible to pull the cables, so you'll want to stay well below that.
3/4" Schedule 80 PVC has a total area of 0.409 in.², of which two 12/2 with ground NM-B cables will fill 65% of. Depending on how long the sleeve is, this might make it quite difficult to pull the cables through.
Fill = Conductor Area / Total Area
Fill = (0.132025 * 2) / 0.409
Fill = 0.26405 / 0.409
Fill = 0.6456 = 65%
To put this in perspective. If this was two wires in a complete conduit system, you'd only be able to fill to 31% of the total area of the conduit (0.127 in.²).
Wet and Damp Locations
If this area can be considered damp or wet, you'll have to use cable rated for wet/damp locations. In this case, you'll want to use UF-B cable (463 mil major diameter). Two of these cables will fill 82% of the conduit, making it nearly impossible to pull by hand (depending on the length).
Fill = (((π * ((463 / 1000) / 2)²) * 2) / 0.409) * 100
Fill = 82.33%
Best Answer
TL;DR -- go right ahead
This looks to be a sound plan -- it's definitely cheaper than running everything in conduit, and probably somewhat easier too even with having to stuff two NM cables down a 3/4" EMT length.
Fill isn't an issue
Even though the Code conduit fill criteria don't apply to protective sleeves, you're fine on fill (albeit barely). Encore Wire quotes .160" by .450" for their 12/2 W/G NM product -- multiplying and converting to mm^2 gives us a conservative area estimate of 46.5 mm^2 per cable. Doubling this to account for the doubled run gives us 93mm^2, which is within the 106mm^2 (31% fill) available for two wires or cables in the 3/4" EMT.
Nor are derates
Normally, since you have more than 3 current-carrying conductors in the sleeve, you'd have to derate as per 310.15(B)(3)(a). However, this derate starts with the 90°C column in Table 310.15(B)(16) as per 334.80. Since we are derating by a factor of 80% for the 4-6 conductor case, we get 24A derated, which is above the 20A 60°C ampacity that's actually used for the cable as per 334.80, so we're good on that front too.
And the EMT is suitable
NEC 334.15(B) states that EMT is one of the allowable means for protecting NM from damage in an exposed application. PVC can also be used, but it must be Schedule 80 PVC -- the cheaper/wimpier Schedule 40 stuff just won't do. There shouldn't be any concerns with impedance, grounding, or the likes either -- protective sleeves are not required to be grounded or bonded as per 250.86 Exception 2, and it's grounded anyway via being connected to a grounded metal box.