How should chuck roast be grilled to maximize tenderness?
Your best method of making chuck tender is to cook it low and slow, as per traditional barbecue.

The reason for this is that chuck is a relatively tough meat, full of connective tissue (the protein collagen, among others).

By cooking low and slow for a long time, you raise the internal temperature of the meat to about 170 F to 180 F for an extended period, at which temperatures over time, in the presence of moisture (either part of the meat itself, or external) the collagen will convert to gelatin, converting from a touch to a succulent and tender product.  This is the principal employed in both braising and barbecue (as opposed to grilling, which is a high heat technique normally).

Note that the collagen to gelatin conversion creates the tenderness, it is not preserving tenderness, since it was not a tender cut to start with.

At this point, since you have already sliced the meat, I believe your best approach would be to create an indirect fire or flame.  Place the meat in a disposable pan layered together to minimize drying, away from the fire, and allow to cook slowly for at a low temperature (say 250 F to 300 F air temperature as measured in the grill with its lid on) for several hours, at least about 4 hours and up to 8 to 12 hours, until it is fall apart tender.

You may also do this in the oven rather than on the grill.  It will also be amenable to braising techniques, as opposed to dry roasting/barbecuing.

While it is too late for this particular chuck, in the future you may have better success not slicing it prior to barbecuing.

If you are thinking of grilling as a high temperature technique and do not want to do the low and slow methods, I am afraid this will not work out well with chuck.  It is naturally not tender, and these high temperature grilling will not facilitate enhancing the tenderness.  If you are time constrained, you would be better off freezing your marinated chuck for another time, and cooking something else that does respond well to the fast, intense heat of grilling, like steaks or pork chops.