1. ...Yes and no? Each of the shops draws semi-randomly from a given pool of possibilities, and they tend to refresh about once every two [in-game] days or so. So any given blacksmith
might sell a crossbow, but they are never guaranteed to have them. That being said, you're also looking at Dawnguard wrong. Assuming that it's installed, you'll start getting fairly frequent vampire attacks between the time you hit level 9 and the time you finish the Dawnguard questline. These vampires can and
will kill non-essential npcs
very quickly[footnote]Which is particularly annoying if they kill the merchants.[/footnote], so unless you're either waiting to install the dlc or are both below level 9 and willing to install the Dawnguard Delayed mod (
For Special Edition,
For Regular Skyrim), you'll probably want to put Dawnguard at the top of your priorities list anyways.
2. Speech is largely a waste. In addition to being a pain to level (I don't think any of my characters have ever gotten it to 100), most of the perks are redundant. For instance, if you do the Thieves Guild questline (including enough radiant quests to restore the guild's influence in the major holds in core Skyrim), you receive the Amulet of Articulation, which allows you to pass every persuade/intimidate check in the game. Additionally, getting the guild to that point allows you to invoke the connection and bribe the guards. So taking the Thieves Guild to completion renders the right side of the Speech tree completely irrelevant. The left side is more useful, it's also entirely a convenience thing which ultimately boils down to you having to fast-travel less to sell your loot to different merchants.
3. Destruction magic's problem is that it doesn't scale. If you want to be a destruction mage you're probably going to want to look into mods like Perkus Maximus, Ordinator, Apocalypse or Midas Magic. That being said, those are all "overhaul" mods, and probably not ideal for a first playthrough. Magic in general really gets shunted in Skyrim, with a good number of perks tied up in "cast spells for half cost" perks.
And yes, Conjuration is
much more effective than Destruction, because its bound weapons (particularly bound sword and bound bow) give you decent quality weapons which scale with the relevant perks. To repeat myself, however: Mages are shunted in Skyrim. Those bound swords and bound bows are
spectacular early game, but if you invest in Smithing then you will easily surpass them by improving late-game equipment at the whetstone.
4. Most content in the game is level scaled. That said, you'd probably want to be level 12+ before starting the Civil War.
5. Yes. You can become a werewolf through the Companions questline, and you can become a regular vampire through combat with a regular vampire. The latter is very chance based and only has 10% a chance of occurring if a) You are not immune to disease, and b) the vampire casts vampiric drain on you.
In terms of commonalities, both Vampirism and Lycanthropy give you immunity to disease.
Vampirism has some perks to it, such as 20-50% resistance to frost (resistance improving the longer you go without feeding), a 25% bonus to your illusion spells and sneaking ability, the vampiric drain spell, a once-per day reanimate dead spell (scaling in a similar manner to your resistance to frost), a once per day calm spell (only available if you go 1+ days without feeding), a functional equivalent of night eye (much more useful if you install lighting mods than in vanilla skyrim, absolutely redundant if you're playing as a Khajiit), and a once per day invisibility power (only available if blood starved). Countering this, Vampirism also makes you 20-50% weaker to fire, and in daylight your health, magicka, and stamina are all reduced by 15-60 points and their regeneration rate is negated.
Lycanthropy, by contrast is almost entirely contained within a once-per day power which lets you transform into...well, a werewolf for 150 seconds. As of Dawnguard, Werewolf has its own perk tree which is advanced by eating the hearts of corpses in your wolf form (to skill it up, you must enter the skill menu when in wolf form). Before taking that into account, however, the wolf form does the following: increases your health by 50 and your stamina by 100. Stamina regen increases from 5% to 20%. Carrying Capacity increases by 2000. Wolves will not attack you. Base unarmed damage increases from 4 to 20, and reach improves from 96 to 150. Your claws have a Fortify Unarmed Damage effect which increases the damage by another 0-60 points (depending on level. 0 below level 11, 60 at level 50+). If Dawnguard is installed you also get damage resistance of 0-400 (same level scaling). Your base rate of health regeneration, however, is set to zero and you cannot use any items, spells, or equipment when in beast form. Additionally, you cannot get the "rested" buff (temporary bonus to skill up rate) for sleeping in a bed at any point if you are a werewolf. Transforming in front of almost any non-follower friendly NPC will also give you a bounty of 1000 at the nearest major hold (which is very handy for getting a steam achievement that requires that you have a bounty of 1000 at every major hold), but if nobody sees you transform you are effectively anonymous.
Of these two, Lycanthropy is far and away the winner simply because basic Vampirism has significant downsides but not much in the way of upsides. That being said, Dawnguard also gives you a few opportunities to become a Vampire Lord, an enhanced strain of vampirism that still holds to all of the rules listed above but also grants an at-will power to transform into...well, a Vampire Lord which is more on-par with Lycanthropy's Beast Form and like Lycanthropy has its own skill tree. Whether or not that compensates for the downsides is fairly subjective. If you want to become a vampire lord there are several options: 1) accept Harkon's gift early in the Dawnguard questline (this counts as joining his faction). 2) ask to become a vampire at the Soul Cairn part of the Dawnguard questline[footnote]Note: If you entered as part of the Dawnguard, you'll need to cure your vampirism after completing the Soul Cairn. Until then, the Dawnguard will reject you and refuse to let you progress in the questline[/footnote], and 3) You can ask to become a vampire at any point after the Dawnguard questline is completed...provided that the one turning you isn't convinced to give up their own vampirism.