in the dark as to how to run elastic under plank for jib sheets.
Any number of ways of doing this. Usually there will either be holes/tubes in the end of the plank, or else turning blocks under the end. Either way the leeward sheet goes across the boat to the tube/block on the windward end of the plank. If there's a turning block underneath then the elastic often goes in a drain hole or something on the bottom of the plank to inside the open area. Either way when you get to the end of the open area the elastic either ties off there or alternatively goes round a turning block and back for more strength.
Also, there is loop of elastic attached to tiller (for centering helm) but, I can find no visible point(s) of attachment to boat.
How long is your elastic loop? If its short and heavy then one system is to have the elastic from the end of the tiller to the leading edge of the fixed part of the stock. SUprisingly this does seem to provide enough self centering and is also nicely self contained so there's nothing to rig...