It’s not easy to guess how it would look if you do the weighting in the restpose. I would recommend:
*) Export this rifle anim (that is already retargetted to your skeleton) as fbx from UE4.
*) Import it in Blender. It should look almost ok (don’t waste your time with the skeleton if it does not - even if it is terrible rotated… you would not improve the skeleton but the weightpainting right now)
*) Unrotate all bones that are not of interesst (select and do ALT-R pelvis, spine_01, …) focus on your arms. We would get a symetric mirror-pose.
*) Select all your arm and hand bones in pose mode, include the clavicle as well and hit “CTRL-C” in pose mode.
*) Now mirror your selected bones pose with Shift-CTRL-V.
*) If you exported the mesh with your animation then remove the mesh from UE4 export now in Blender (you don’t need it… grab an original mesh from Blender).
*) Open a further Blender session. Copy (only) your relevant mesh(es) with CTRL-C and paste it into your second Blender window with CTRL-V.
*) Add the skeleton from the animation as modifier to your copy/pasted mesh(es) (your copied mesh should now be in the pose of the anim).
*) Now… start weightpainting. Autonormalize on. X-Mirror on (or if not then copy all those group in vertex manager). I think it’s faster with X-Mirror on but usually it would make some dirty painting.
*) If you are done… Check your mesh if the X-Mirror did not do weird stuff: In weightpaint mode go to the options tab (left “T” sidebar) and choose “Show Zero Weights”: Active…
*) Go to the vertex groups (triangle symbol right side) and click on all of them (arrow down key on keyboard). If you see a weight on both sides of the arms at one group… remove it with your subtract brush in weightpaint mode.
*) If you are done… click to the Tools and Weight-Tools tab in Weightpaint mode (left “T” sidebar) and do “Limit Total” and choose 4 or 5. Usually 4 bones should be enough for any vertex (and should remove some dirt from X-mirror… like 0.00001 on lower_arm_l from lower_arm_r … which is garbage).
*) Afterwards do… “Clean” in the Weight-Tools tab to remove all 0.0 weights. If a vertex does not got weight on a bone then the assignment is useless.
*) Afterwards do… “Normalize all” in the Weight-Tools tab… to make sure that really every vertex got 1.0 weight.
*) Copy your mesh and put it back to your second Blender window (save and remove the original mesh first). Assign the armature modifier back to your orignial skeleton (that one that was no UE4 import but some MH import modified by you few days ago or so).
*) Take a look into the materials … if you copy paste via two Blender windows Blender might think he have now 2 materials and renames something to material.001 or so (rename it back … as you deleted the original mesh).
*) Now export the mesh from Blender again and do Reimport mesh in UE4. You should have the exact same looking pose and the exact same looking weightpainting (as in Blender).
Regarding your cloth. Weight transfer is the correct way to go with this. However you could try to fix this issue with different transfer options. E.g. if it should take the nearest vertex or face. It’s even possible that you got too less polys at the ellbow (try subdiv). For example if your cloth ellbow is green with few faces and your skin ellbow is green with many faces the cloth would do it’s best via splitting up the 0.5 between is few faces but it would look like you would create a circle with just 2 matches vs a circle with 10 or so. And as a further hint… those polys are hidden (right now). If you don’t plan to show her naked ellbow then remove those skin polys. You would lower the polys and with this suit it should even be not that necessary (while I still recommend to add) the weightpainting for the twistbones (as the arm/hand would usually turn below the cloth). This blue cloth is no knights-armor of course but it’s even no rubber and it would not look that painfull like a twisted balloon without any cloth with offset (like in the lower screenshot) if you keep the hand and the lowerarm_l weights seperate.