You're growing cigarettes B3?
They're not hardy so I wouldn't start yet. No rush, we've a bit of time yet before summer
In the sun it would cook, so needs to be in shade.
Overnight, much the same temperature as outside.
What are you going to sow?
I don't think hard pruning but not into old wood is a possiiblity.
I haven't had much success with cutting back leggy hebes, one looked OK for a couple of years, lots of new growth from the base, then snuffed it.
I find seed compost is generally a bit hard and mix it with some MPC and sand/grit to free it up a bit.
I haven't tried the Homebase version
Other than primula and auricula, what plants have you got inside. Most hardy plants don't like being inside.
If mine get very crowded they stop flowering. Maybe splitting up would help
If they germinate they'll be healthy plants. If they've been stored cool and dry they should be OK to germinate
The cultivars like 'Ruby Cloud' don't seed reliably true but you should get some nice ones even so.
agree with Lyn re cold