You cannot use the net like this
The battle master maneuver affects your reach with a weapon. This is a property separate from the range of a weapon. The net, when used for a ranged attack, uses its own range, but is unaffected by your reach, which only comes into play when you make a melee attack (see PHB 195).
You could, technically, apply the maneuver ...
Lunging Attack can only be used on melee attacks, and nets are ranged weapons
The description of the Battle Master fighter's Lunging Attack maneuver says ((PHB, p. 74; emphasis mine):
When you make a melee weapon attack on your turn, you can expend one superiority die to increase your reach for that attack by 5 feet. If you hit, you add the superiority ...
The attacker should roll a Stealth check with advantage.
The burrowing monster is hiding (under the ground) and heavily obscured (because the ground is in the way). Perception checks to detect a heavily obscured creature have disadvantage.. You can either let the burrower roll Stealth with advantage or give the victim -5 on their passive Perception to ...
You can roll a stealth check against their passive Perception -- but don't have to. You, the DM, decide when a character is surprised:
The DM determines who might be surprised.
Yes, your monsters would qualify as unseen attackers if they are hidden (should require a Stealth check), and gain advantage on the first strike.
When a creature can't see you, ...
So, a lance, of course, has the following text:
You have disadvantage when you use a lance to attack a target within 5 feet of you
We are considering two different fixes to the (lack of) mounting rules:
You occupy the same space as your mount
You don't, but occupy grid-space your mount also occupies
(these are the fixes from what used to ...
One more method: reroll
One rapid method I've used as a DM (or Druid handling eight wolves), using average damage:
Roll a single d20 for all attacks, then reroll misses.
While this fails to calculate the possibly of extra crits on the initial hits, that too can be accommodated (if desired) by rerolling the misses off to the left, and rerolling the initial ...
I ran some calculations to show what this might look like, the first set of numbers is what I called "Pooled" Advantage where you take the 6 highest dice of 12d20, the second is "Standard" Advantage where you take the highest values from 6 pairs of d20s.
I used python to do 100 million rolls of each variant. The results are without modifiers, with critical ...
Rolling 12d20 and taking the 6 highest is not equivalent
Taking the 6 highest rolls out of 12d20 is not equivalent to 6 attacks with advantage. It is significantly better. I could lay out the probability argument, but it's easier to cherry-pick an example that demonstrates the difference. Let's say you roll your 6 attacks with advantage. On the first 3 ...
I would say, if I was DM, that you would have to use separate rolls or some differentiation of dice pairs.
For instance, if you rolled at advantage for six attacks, you could get a result on the dice of 20 and 12 on the first attack, 18 and 14 on the second, 1 and 4 on the third, 12 and 7 on the fourth, 20 and 1 on the fifth, and 15 and 17 on the sixth. ...
No, it does not allow Advantage on all attack rolls
This is spelled out quite explicitly in the rules you quoted:
Starting at 1st level, you can manipulate the forces of chance and chaos to gain advantage on one attack roll, ability check, or saving throw.
So you did right the first time.
As Ryan Thompson has pointed out in his answer, this ability does not specify any limits or other prerequisites other than that your target has "one or more of its allies within 5 feet of it." This means that every turn, you gain advantage against such creatures.
This is in line with a paladin's level 7 feature. Most other paladins gain a passive aura that ...
Cull the Herd applies to every attack as long as the conditions are met
The ability doesn't specify how long the advantage lasts because it doesn't have a duration or a limited number of uses. Each time you make an attack, if the conditions are met at the instant you make the attack, then the ability applies and you have advantage for that attack.
Sacred weapon would create bright light for a 20 ft radius:
As an action, you can imbue one weapon that you are holding with positive energy, using your Channel Divinity. For 1 minute, [...] weapon also emits bright light in a 20-foot radius and dim light 20 feet beyond that.
--- Player's Handbook, p. 86
Meaning any creature within 20 ...
No attacks are made with advantage.
Hypnotic Pattern states:
On a failed save, the creature becomes charmed for the duration. While charmed by this spell, the creature is incapacitated and has a speed of 0.
The charmed condition stated:
A charmed creature can't attack the charmer or target the charmer with harmful abilities or magical effects.