Itinerary for Hokkaido in Feb

I will be going to Hokkaido for 7 days from 5 Feb to 10 Feb. Planning to visit Sapporo (2 days) and Otaru (1 day). But would also like to visit Lake Kotan and Lake Masshu, and the hot spring at Noboribetsu.

What would be a good place to stay to use as a base to visit Lake Kotan and Masshu?

I'm wondering if it would also be possible to squeeze in a trip to northern Hokkaido (Horonobe) to visit the reindeer farm. Will there be enough time to do so?

Seems to me from my trip to Hokkaido several years ago that you will lose a day each way to Eastern Hokkaido on the train. Same to Northern Hokkaido. It's a big island and the trains take all day.
Suggestions: visit Daisetsuzan Nat Park in the center of Hokkaido. There are hotels and hot springs there. It can be reached from Sapporo in 1/2 day.
Noboribetsu is also a nice place to stay, the hot spring area looks like a Swiss village. Ryokan Hanaya is a short walk from a very large onsen. It is also about 1 hour by train (from Noboribetsu) to Lake Toya and Shikoysu-Toya Nat Park. Mt Usuzan, a very active volcano, has a cable ropeway to the rim of the crater and beautiful views of the Nat Park. I stayed 5 days in Noboribetsu and 5 in Sapporo and found plenty to do and see from those 2 bases.

Technically, neither Lake Toya nor Shikostru are on a train line. Each is a bus trip away from the nearest station.

On a map it looks like Noboribetsu is better linked to Toya by road.

Not sure about the easiest connection between Nobori and Shikotsu (or btwn the 2 lakes for that mattter).

And as for Daisetsuzan, how can one find out which parts are accessible in mid-winter and which ones aren't?

Finally, if seeing the drift ice is part of your winter plan, consider getting a flight from Sapporo to Memanbetsu, and staying at either Abashiri or Utoro (Shiretoko peninsula). That would get you to the NE of Hokkaido quickly.

Tama is correct, you must take a bus from the JR Toya train station to get to Lake Toya but it is a very short ride, maybe 30 min. The road from Noboribetsu to Lake Toya is very scenic but long and winding and the bus is not JR (no JR pass accepted) nor does it run as often as the train.
There is a beautiful gorge with waterfalls near Sounkyo Spa in Diesetsuzan but I went in the fall, not winter so not sure about mid winter access to other parts of the park. The hotel I stayed at in Sounkyo had an outdoor hot spring pool on the roof - the view was beautiful and stayed in the pool for hours each day. Thereare ski lifts in Sounkyo.

I take it you are going for the yuki matsuri? If not, definitely check it out. I went back in 2002 and it was pretty cool. Make sure to go to it as early as possible though because the ice sculptures seem to degrade a bit as time goes on.

And dress warmly! I am from the southern USA so I had never been in weather like that... I am glad I ordered a bunch of layered winter-wear from an outdoor sporting goods store. I had on sock liners, wool socks, and hiking boots... my toes were still half-frozen.

Oh, and a tip... there is ice everywhere up there. They sell little slip on spikes for shoes at most shops it seems. Definitely buy some! The pair I got were less than $10... rubber straps with some little round metal studs on the bottom to grip the ice.