I've had 4 day old chicks running around in 50°F temps, they don't mind it at all as long as they can get back to their heat source (hen or Mama Heating Pad, or in your case, heat plate) when they start to get cold.
From the bottom of what? And, IMHO, NO on the heat lamp. They have their heat plate, chicks don't need the "standard" ambient temps of 95° the first week dropping by 5°F weekly until they are feathered.
That might be necessary for large scale farm raised chicks but they typically use hover brooders which are similar to a heat plate with an AREA of the brooder space being heated higher than ambient. They aren't going to heat an entire barn to 95°F
What chicks need is a proper heat source they can go to when they get cold.
Don't bother. As with a MHP, watch the chicks. If they won't come out from under the heat plate it is too cool, if they won't go in it is too hot. If they come and go, sleeping under it, the plate temp is fine. You will need to raise it as the chicks grow (which is FAST!)
I've never done chick grit. They don't need it if they ONLY get chick starter. Mine have always been on dirt (barn alley and outdoors) where they seem to find plenty of "grit". Now if yours are in a "clean" environment and they are getting eating some things that is not commercial chicken food, I think having it separate would make it so they eat as much as they need. But it may not matter if it is mixed in with the food, they'll pick out what they want. I have oyster shell on the side for my layers.