After Plockton, Which Route?

After Mull, are you going to Skye via Ardnamurchan/ the Mallaig ferry?

Plockton is a lovely wee place but it's very close to Skye so not sure you need another night there though there are various places to eat out and it has a pretty waterfront.If then going to Newtonmore (via the Spean Bridge option) Glengarry Castle hotel is a place we often stop at - pricey for lunch but lovely for tea/coffee and cake/scones.Nice short stroll down through the gardens to Loch Oich. I used to love the Pottery Bunkhouse Cafe near Laggan but it was taken over a year or two back and has gone too vegan for me.
You'll be driving through better scenery again if you make for Ullapool though it's a very long way if you hug the coast road all the way up through Torridon (magnificent area.)That is the most scenic route to Ullapool, instead of heading inland to Achnasheen.
In fact ,if you take in the Bealach na Ba mountain pass to Applecross then round the coast to Torridon that would be where I would break my journey.It's a drive unequalled in Scotland IMO. Very different scenery to Skye and where else you have been.
However, getting accommodation in Torridon for one night is not easy!
On route, the Applecross Inn is excellent for lunch; the nearby Walled Garden is nice too.
The stunning Torridon area has Jo's Cafe, the Wee Whistlestop Cafe and the Shieldaig Inn.
It is possible after Torridon to cut out the rest of the coast road up through Gairloch, if Ullapool is drawing you. It's an attractive port; two nights would allow some exploration of the fantastic Coigach/Assynt area to the north before you head for Newtonmore.
Newtonmore is nothing special in itself though some time to see something of the Rothiemurchus area adjacent to Aviemore, showcasing some of the best of Cairngorm National Park , is certainly worth time.

Plockton is only an hour or so from Portree, whereas Plockton to Ullapool is the better part of a day, depending on the exact route and where you stop, so I would have thought that Skye to Ullapool with a stop in Plockton was the better option.
To me Ullapool is a one night stop unless you are going to do part of the NC500 (and back) to Lochinver and Kinlochbervie, maybe as far as Smoo Cave and Balnakiel (Durness).
Or maybe the boat trip to the Summer Isles.
If going from Ullapool to Newtonmore rather than going South via Garve I think the better route is the A837 (Oykel Bridge) to the Shin Falls, Lairg, Dornoch and Tain. Coming out of Lairg it's only a short diversion to Dunrobin Castle.

If going from Plockton to Newtonmore my own preference would be southabout. But then northabout takes you along the Loch and past Urquhart Castle, so it very much depends.
South you have the gorge and waterfalls between Spean Bridge and Tulloch (I would suggest best seen from the train) and the Commando memorial. Beyond Tulloch you have classic highland scenery up through Laggan and you could divert to the Dalwhinnie distillery.
Beyond Spean Bridge not a lot of eating options except the Dalwhinnie Hotel and Ralia.
But then Ralia is only a couple of minutes from Newtonmore.

We will actually stay 2 nights in Mallaig prior to catching the ferry to Skye (I forgot this stop in my original post!). Is Newtonmore a good base for day trips to Culloden and areas north? Or south (tho we head that way en route to Crail).

Your two nights in Mallaig don't really affect this part of the itinerary. One of those days will be the transfer from Mull, especially if you include the divert to Glenfinnan.
The other I would see as being the wildlife cruise or Knoydart. Or, and it is a developing situation, some possible unusual opportunities for day trips to the Small Isles. Maybe the beaches at Morar.
I don't know what route you are taking to Crail from Newtonmore- you could blast through In about 2 hours down the A9.
But from Newtonmore Culloden should be around 45 minutes each way, the Black Isle around 90 minutes. The Highland Folk Museum is next door, the Cairngorms and Grantown are 30 to 45 minutes each way, longer if you add in the odd distillery or two.
Heading south you could have a wonderful day with some combination of Blair Atholl, Pitlochry, Dunkeld, Loch Tummel and the Crannog Centre.
Equally you could do some or all of that on the way down the A9 to Crail

Personally I would see you going to Crail via Grantown, Tomintoul, Braemar and Glenshee, possibly adding Glamis Castle if you had time (not at all sure you would)- that has to be the best way to do the trip.

You have a smorgasbord of offerings and lack the time to consume the whole smorgasbord so there are hard choices to make.

You can easily visit Culloden as you drive south to Newtonmore as its barely a 90 min drive away from Ullapool.
If coming from Plockton, it's around 2 hrs to Inverness and another 15mins to Culloden.
No need in either case to see the battlefield site from Newtonmore unless you spend all day exploring what's on the driving routes from your west coast Base.
As said, the Highland Folk Museum is at NM and plenty of fab Cairngorm scenery around Aviemore....unless you have other specific interests re visits to sites/sights.