Take te axle out and redo the spacing to the drive side with thinner lock nut to the non drive side and you should have enough space for the 7 speed freewheel. 2mm on the drive is nothing to worry about for a well maintained wheel. QR axle if not properly adjusted will break. most QR axles are cromoly or heat treated and solid axles aren't, although wheels manufacturing does make a solid cromoly axle if needed.